ETF and Index Fund Glossary: NAV, Creation Units, Tracking Difference and Costs
Exchange-traded funds and index funds are pooled investment vehicles, but an ETF’s traded share price, net asset value and underlying exposure are not always identical. Understanding the mechanics of ETF creation, redemption, benchmark replication and trading costs helps explain what a fund actually delivers.
This ETF glossary clarifies the terminology found in fund factsheets, exchange listings and benchmark methodology documents. It distinguishes the index calculation from the investment product and the fund’s own returns.
What this terminology helps you understand
A correct definition is only the starting point. Analysts must identify the relevant instrument, reporting date, measurement method, currency, comparable reference and limitation before drawing conclusions. Similar labels can describe different calculations in different datasets, financial products or jurisdictions. This guide therefore pairs each term with its practical use and a reason not to overinterpret it.
ETF and Index Fund Glossary: definitions and analytical context
Exchange-traded fund (ETF)
A collective investment vehicle whose shares ordinarily trade on an exchange during market hours. The legal form, eligible assets and investor protections depend on jurisdiction. ETF shares can trade at premiums or discounts to the value of underlying holdings, especially when underlying markets are closed or less liquid.
Index fund
A pooled investment strategy intended to track a specified benchmark rather than make unconstrained discretionary security selections. Some index funds are ETFs and others are non-exchange-traded mutual funds. Methodology, fees and tracking outcomes vary.
Benchmark index
A published calculation based on eligibility, weighting, rebalancing and return conventions for a selected universe. An index itself is not a purchasable asset. An investable portfolio must face fees, execution and taxes that can create performance differences.
Net asset value (NAV)
The fund’s calculated assets minus liabilities divided by the relevant units outstanding at its valuation point. Published NAV is not always current during exchange trading. Stale underlying prices, currencies or valuation conventions complicate comparisons with live quotes.
Indicative NAV (iNAV)
An estimated intraday per-share reference calculated from available price inputs under a provider methodology. It may be stale or unreliable if underlying holdings are closed, illiquid or valued with proxies. It is not an executable bid.
ETF premium
The percentage by which the ETF’s market price exceeds a chosen NAV reference. A premium can reflect time-zone differences and price discovery rather than pure pricing error. The NAV timestamp must be checked.
ETF discount
The percentage by which market price falls below a chosen NAV reference. A discount is not automatically an arbitrage opportunity for a retail investor. Transaction costs, restricted access and valuation uncertainty may prevent convergence.
Authorised participant (AP)
An institution with an agreement permitting direct ETF share creations and redemptions in specified creation-unit sizes. APs can support the mechanism connecting ETF supply with underlying holdings, but they are not obliged to eliminate every price divergence instantly.
Creation unit
A prescribed large block of ETF shares used for direct transactions between the fund and authorised participants. Retail investors usually trade ordinary shares in the secondary market rather than submitting creation or redemption orders directly.
Creation basket
The specified securities, cash or combination delivered for a creation under the fund’s rules. A basket may differ from a complete pro rata copy of holdings. In-kind and cash arrangements, transaction fees and substitutions matter.
In-kind redemption
Delivery of securities instead of only cash when an AP redeems ETF shares. It can change the fund’s trading and tax profile but depends on fund structure and local rules; it is not available to every shareholder.
Full replication
A strategy that holds all index constituents at approximate benchmark weights where practicable. It can be expensive for broad or illiquid indices, and corporate actions still require operational adjustments.
Sampling
A strategy holding a representative subset of a benchmark’s securities to approximate key risk exposures. Sampling introduces modelling and selection risk. Performance may differ even before fees and trading costs.
Synthetic replication
A structure using derivatives such as swaps to obtain target index returns rather than only holding underlying constituents. Counterparty exposures, collateral quality, swap pricing and legal structure require separate analysis from conventional physical replication.
Tracking difference
The realised fund return minus the benchmark return over a defined period, including fund costs and methodological differences. Tracking difference is directional; it is not the same as variability of tracking returns. Compare like-for-like return types and currencies.
Tracking error
The variability, commonly annualised standard deviation, of the fund’s return differences relative to its benchmark over a specified sample. A low tracking error does not mean the fund has no consistent fee drag. Sample frequency, window and benchmark definition matter.
Ongoing charges figure (OCF)
A standardised or jurisdiction-specific summary of specified ongoing fund expenses expressed as a proportion of assets. It may omit trading costs, spreads, borrowing and some other charges. Read the prospectus and relevant fund disclosures.
Bid–ask spread
The difference between the displayed buying and selling quotes for an ETF share. Realised trading cost can exceed the quoted spread because of order size, market impact, commissions and changing underlying liquidity.
Accumulating share class
A fund share class that generally reinvests income within the fund rather than distributing it to the holder. Tax treatment and reporting requirements depend on jurisdiction and account type, not solely on whether cash is distributed.
Distributing share class
A fund share class that pays eligible income to holders under its distribution policy. Distribution yields are not guaranteed returns, and a payment usually reduces fund value correspondingly at the relevant point.
Currency-hedged share class
A share class intended to mitigate specified foreign-exchange exposure using hedging instruments. It does not generally remove all currency effects, costs or basis risk, and it may hedge a different exposure than the investor expects.
Securities lending
The temporary lending of portfolio securities under collateral and counterparty arrangements, sometimes generating fee income. Income may offset some expenses, but it introduces counterparty, collateral, operational and legal risks that vary across funds.
Worked educational example
Assume a fictional ETF’s live mid-price is £102.40 while its latest published NAV is £102.00. The simple quoted premium is (£102.40 ÷ £102.00 − 1) × 100 = approximately 0.39%. If the underlying foreign market closed hours earlier, however, the reported NAV may not reflect developments since its valuation. A 0.39% displayed premium is therefore not proof of a 0.39% overpayment relative to current fair value. For another example, a fund returning 7.4% when its comparable benchmark returns 7.8% has a tracking difference of −0.4 percentage points. That says nothing by itself about the standard deviation of the fund’s daily active returns, which determines tracking error.
Comparison table: choosing the right measure
| Term | What it tells you | What it does not tell you |
|---|---|---|
| NAV | Fund value at a valuation time | Current executable market price |
| Premium/discount | Market quote relative to NAV | Guaranteed arbitrage value |
| Tracking difference | Realised relative return | Variability of relative returns |
| Tracking error | Consistency of benchmark tracking | Direction of tracking shortfall |
How to apply these terms in responsible market research
Begin with a clearly stated question rather than selecting a metric because its recent reading looks attractive. Identify exactly what the figure measures, which inputs are observed and which are estimated. Record the source and timestamp; compare the value with a relevant peer, prior period or stated benchmark; then test a plausible alternative explanation.
When an analysis involves a model, perform a basic sensitivity check. Change one assumption at a time and explain how the interpretation changes. Distinguish correlation from causation and a historical sample from a forward-looking expectation. Reporting conventions, contract terms, fees and liquidity can make seemingly similar figures economically different.
Finally, write a short limitation statement. Ask what the measure leaves out, how the data could be distorted in a stressed market and what evidence would contradict the initial conclusion. A transparent analytical process is more useful than an apparently precise number without context.
Worked interpretation: tracking a benchmark is not the same as matching investor returns
Imagine a benchmark gaining 8.0% over a year and a fund reporting 7.65% on a compatible total-return basis. Its tracking difference is −0.35 percentage points. That difference could arise from fees, withholding taxes, index replication, cash holdings or trading costs. It does not show how variable the fund’s return shortfall was through the year; that would require a sample of periodic active returns to estimate tracking error. A fund can trail consistently by a modest amount yet exhibit low tracking error.
The actual investor may experience another result. Purchasing ETF shares at an offer and later selling at a bid incurs an executable spread. Commissions, currency conversion and the exchange’s dealing times can add cost. If the underlying market is closed, an ETF may update its price in response to fresh information while the most recently published NAV remains based on older prices. Treating this as a certain arbitrage opportunity would ignore timing and execution risk.
Fund factsheets also require like-for-like comparisons. One index may include dividends, another may present only price returns, and two nominally global indices may use different country classifications or free-float weighting rules. A share-class currency is not necessarily the currency of economic exposure, while a currency hedge does not eliminate every currency effect. Check benchmark version, replication method, tracking data, borrowing or lending disclosures, tax treatment and total ownership cost before concluding that two products provide identical exposure.
Common interpretation mistakes
- Confusing fund-level benchmark exposure with the specific risks of CFDs, futures or other products referencing that benchmark.
- Using a headline OCF as an estimate of every trading and ownership cost.
- Assuming an ETF trading during a foreign market holiday has real-time prices for all underlying holdings.
Frequently asked questions
Are all ETFs passive?
No. ETFs can follow an index or be actively managed. The trading structure and investment objective are separate features.
Why can an ETF price differ from NAV?
Its shares trade continuously while NAV is calculated using a specific valuation process. Liquidity, time zones, market makers and information changes contribute to differences.
Does a low fee guarantee good index tracking?
No. Benchmark design, taxes, sampling, securities lending, market access and operational costs also influence tracking outcomes.
Related Traders MBA education
Continue with Stock Index Weighting Explained; Index ETFs vs Index Funds and CFDs; Index Concentration Risk or browse the Education Hub and the Trading Glossary category.
Sources and further reading
Important notice: This resource provides general financial education, not personalised financial advice or a recommendation to trade, hold or sell a financial instrument. All numerical scenarios are hypothetical. Market, liquidity, counterparty and leverage risks can cause substantial losses.