Market Microstructure Glossary: Order Flow, Tick Size, Depth and Market Making
Market microstructure describes the rules and mechanisms through which trading interest becomes quotes and executed transactions. A market price is not simply a number on a chart: the venue’s queue, minimum price increment, available liquidity and routing rules affect actual execution.
This market microstructure glossary explains the specialist vocabulary used by exchanges, trading desks and market data providers. It is deliberately distinct from a basic order-types glossary by focusing on matching, liquidity measurement and execution quality.
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.
Market Microstructure Glossary: definitions and analytical context
Central limit order book (CLOB)
A matching mechanism that organises eligible buy and sell orders by stated prices and applicable priority rules. It provides visible resting interest but does not disclose every hidden or off-venue order. The exact matching algorithm differs by market.
Best bid
The highest current eligible displayed buying price under the relevant market-data scope. The best bid may support only a small quantity and is not a guarantee that a large sell order executes entirely at that price.
Best ask or offer
The lowest current eligible displayed selling price under the chosen scope. Quotes can change before an incoming order is matched. Displayed availability is time-sensitive and subject to venue rules.
Tick size
The minimum permitted price increment for a particular security or price band under venue rules. A tick is a price step, not a universal cash gain. Minimum increments affect spreads, queues and feasible quote improvement.
Bid–ask spread
The difference between best offer and best bid at a point in time. The displayed spread understates market impact for orders that consume several levels and can widen abruptly around news.
Order-book depth
The available quantities quoted at specified prices or within a stated price distance from the midpoint. Displayed depth is not guaranteed executable depth; cancellations and order-book fragmentation matter.
Price–time priority
A matching rule giving better-priced orders priority and, among equal-priced orders, earlier accepted orders precedence. Some venues use allocation or pro-rata variants, so queue modelling requires actual exchange specifications.
Queue position
A resting order’s relative priority among eligible orders at the same price and under the venue’s rules. Being shown at the best bid does not mean a limit order will fill quickly. Queue position can worsen if an order is modified.
Maker liquidity
A resting order that adds eligible liquidity under a venue’s definition, potentially attracting a distinct fee or rebate. Not every passive order is guaranteed maker treatment; post-only and matching logic differ.
Taker liquidity
An incoming order that executes against resting interest and removes liquidity. Taker orders may obtain immediacy but face spread, fee and market-impact costs.
Maker–taker pricing
A venue fee design that can differentiate between liquidity provision and removal. Rebates and fees may affect routing incentives and displayed spreads. The economically best fill is not determined by fees alone.
Marketable limit order
A limit order priced so that it can trade immediately against available opposing quotes, subject to its limit. It can be partially filled or remain unexecuted when displayed liquidity is insufficient. Unlike an unrestricted market order, its limit bounds the price.
Immediate-or-cancel (IOC)
An instruction to execute available quantity promptly and cancel the rest under the applicable rules. It does not guarantee any fill, full size or the absence of market impact.
Fill-or-kill (FOK)
An order condition requiring the full quantity to execute promptly or otherwise be cancelled. It can result in no transaction even if part of the book could satisfy a smaller order.
Post-only
An instruction intended to prevent immediate liquidity-taking execution under venue rules. The order may be rejected or repriced depending on the exchange; verify implementation rather than assuming identical treatment.
Iceberg or reserve order
An order structure displaying only part of the total intended size while reserving additional eligible quantity. Refresh rules and priority treatment differ by venue. Visible size is therefore not necessarily the full quantity available.
Dark liquidity
Trading interest not fully displayed in a public lit order book, including specified off-exchange or non-displayed arrangements. Dark does not automatically mean illegal or cheaper. Access, reference pricing, rules and information leakage vary.
Market impact
The price effect or execution cost arising from an order’s size, urgency and interaction with available liquidity. Impact is nonlinear and depends on prevailing conditions; an average day’s volume does not guarantee capacity during stress.
Slippage
The difference between an intended or reference execution price and actual fill price under a specified comparison. Include commissions, spread and appropriate benchmarks when reviewing full implementation shortfall. A favourable slippage reading can arise from market movement.
Adverse selection
The risk that a quote or resting order is executed disproportionately when counterparties have information or prices are moving against the liquidity provider. It is a central cost of making markets, contributing to spreads and quote cancellation around information events.
Consolidated quote
A composite of eligible bids and offers from relevant venues under applicable market-data rules. It may not include every venue, hidden order or executable price for a specific client; jurisdiction and entitlement matter.
Order routing
The process by which a broker or algorithm selects venues and manages child orders for execution. Routing incentives, latency, market conditions and instructions influence fill quality. An order submitted to a broker is not necessarily executed on a single exchange.
Implementation shortfall
A benchmark-relative measure of trading cost that compares a decision or arrival-price reference with actual trades and unfilled residual exposure. It can include delay, explicit fees and opportunity cost. The benchmark and treatment of non-execution must be disclosed.
Worked educational example
Imagine a fictional order book with 100 shares offered at £10.00, another 200 at £10.02 and 300 at £10.05. A marketable purchase for 250 shares would take 100 at £10.00 and 150 at £10.02, if no other book events intervene. The quantity-weighted execution price would be (100 × £10.00 + 150 × £10.02) ÷ 250 = £10.012. The displayed best ask of £10.00 was not the achievable price for the whole order. Commission, routing latency and price changes may add further cost; the illustrative book assumes all stated liquidity remains available.
Comparison table: choosing the right measure
| Concept | What it captures | Why it can mislead |
|---|---|---|
| Best quote | Top displayed price | May be tiny size |
| Depth | Quoted quantity away from top | Can disappear |
| Queue position | Fill priority | Depends on venue rules |
| Market impact | Effect of order demand | Nonlinear and unstable |
| Slippage | Deviation from reference | Benchmark choice matters |
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: spread, depth and actual order cost
Suppose a fictional share displays a £9.98 bid and a £10.02 ask. The midpoint is £10.00 and the quoted spread is £0.04. Purchasing 1,000 shares at £10.02 would create £20 of price concession relative to the midpoint before explicit fees, assuming that the full size can actually trade there. Buying at the ask and immediately selling at the bid in an unchanged market would imply £40 of round-trip spread cost. Neither number includes brokerage charges, market impact or price movement.
Now assume the offer contains only 100 shares at £10.02 and a further 900 at £10.06. Executing all 1,000 shares by walking those visible levels would produce a weighted average of (100 × £10.02 + 900 × £10.06) ÷ 1,000 = £10.056. A calculation using only the best quote would understate cost. Real orders may encounter additional cancellations, hidden liquidity, exchange routing and delays before execution.
Price–time priority means a displayed order at the best bid is not automatically at the front of the execution queue. Earlier orders at the same price may have priority, and some venues apply different allocation rules. A maker quote can earn a fee rebate but still face adverse selection if informed trading hits it just ahead of an unfavourable price move. Fee schedules alone do not establish the best execution outcome.
Review execution with a chosen benchmark: decision price, arrival price, midpoint or volume-weighted price serve different questions. Record order instructions, timestamps, partial fills, unexecuted quantities, venues and commissions. Avoid using a single calm-session spread or volume statistic as evidence that a larger order could have been executed cheaply during a market shock.
Common interpretation mistakes
- Treating one exchange’s displayed book as a complete view of fragmented market liquidity.
- Assuming a limit order guarantees execution simply because it joins the best quote.
- Using quoted spreads alone as a proxy for complete round-trip trading costs.
Frequently asked questions
What is the difference between liquidity and trading volume?
Volume counts executed trades over a period; liquidity concerns how much can be transacted promptly and at what price concession.
Is a market order always filled instantly?
No. Exchange halts, market hours, routing restrictions, missing counterparties and broker rules can delay or prevent fills.
Does a narrower spread mean better execution?
Often it reduces one cost component, but queue access, depth, impact, fees and price movement also determine realised execution quality.
Related Traders MBA education
Continue with Level 2 Order Books Explained; How Market Making Works; How to Measure Trading Slippage 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.