Direct answer
In forex usage, “slippage” usually means the difference between the price you expected to get when placing an order and the price you actually received when the order executed. “Slippage questions” therefore focus on whether that execution gap exists, how big it was, and what execution conditions contributed to it. Related concepts such as spread, fill price vs quote price, liquidity, market impact, requotes, and execution delay each explain a different part of the execution chain, but they are not the same thing as slippage.
A useful way to stay accurate is to link each concept to its canonical owner:
- Expected vs executed price gap → slippage
- Quoted bid-ask width → spread
- When and what your order matched/filled → execution details (including delay)
- How trading moves prices while you trade → market impact
Because this topic is sensitive to changing market conditions and implementation details, the only dependable approach is to verify using your own order and fill records rather than assuming stable historical relationships.
Mechanics: definitions and what each concept measures
Slippage (execution price difference)
Slippage is a realized difference: once an order is filled, you can compare the price expectation at decision time (often a quote you saw, or an internal reference) with the actual fill price. For example, if a trader submits an order while the mid or a quoted price is at a level, but the fill occurs after price movement or after order-book changes, the realized fill may be worse (or sometimes better) than the reference.
Key mechanics to separate:
- Reference: what price you treated as “expected” (e.g., last quote, bid/ask at the moment of submission, or a displayed price).
- Execution: the actual fill price and time.
- Gap: expected reference minus executed fill (sign matters).
Without specifying the reference, “slippage” can become ambiguous.
Spread (quoted bid-ask difference)
The spread is the difference between the bid and ask prices you see in quotes. Spread affects trading costs at the time of execution, but it is not, by itself, slippage. Spread describes a property of quotes; slippage describes a gap between expected and actual execution.
If you place a trade at a quoted level and get filled exactly where the quote indicates you should, you can have a spread cost but no slippage. Conversely, slippage can occur even if the spread is steady, when execution timing and liquidity change.
Quote price vs fill price (what “execution” actually references)
A related point is distinguishing the quote price (what the market reports at a moment) from the fill price (what your order actually matches at execution time). Slippage questions often arise because quote and fill are separated in time and may differ.
This separation is where execution mechanics matter:
- Latency/delay: time between order submission and matching.
- Order-book changes: displayed prices can update quickly.
- Matching rules: how your order is matched against available liquidity.
Requotes, partial fills, and execution delay (execution-chain behavior)
Some execution outcomes modify what you can compare:
- Execution delay changes which quote (or order-book state) applies at fill time.
- Partial fills can produce multiple fill prices; the “slippage” you compute depends on whether you compare a single reference price to an average fill, or compare per-part fill.
- Requotes/price checks (terminology varies by implementation) can change the reference point after your order is submitted.
These are different from slippage, but slippage is often the observed result when these behaviors lead to an unfavorable expected-vs-filled mismatch.
Market impact and liquidity (why prices move while you try to trade)
Even in the absence of large headline moves, trading size relative to available liquidity can contribute to price changes that occur during your order’s path from submission to execution. This is commonly discussed as market impact and liquidity effects.
Market impact is about how your activity and order interaction influence price. Slippage is about the measurable execution gap for your order. Market impact can be a cause of slippage, but slippage itself is the outcome.
Evidence or example: bounded comparisons you can compute
Example 1: Spread cost vs slippage gap
Assume you observe bid/ask and submit a buy where you reference the ask price you see. If the order fills at that same ask price, you experienced the spread cost, but your slippage gap (based on that reference) is approximately zero.
If the order fills later at a higher ask (or after your reference changed), then the execution gap is slippage. The spread may also have widened or stayed constant; the slippage question is about the expected-vs-executed price comparison.
Example 2: Delay causing execution mismatch
Assume price is moving slowly. You submit an order at time T using a displayed quote as the expected reference. If execution occurs at time T+Δ after the relevant price level shifts, your fill can differ from your reference even if the spread at T and T+Δ is similar. In this setup:
- Execution delay / timing is the mechanical reason for reference mismatch.
- Slippage is the measured difference.
Example 3: Partial fills require clear measurement rules
If an order fills in parts at different prices, you must define slippage measurement:
- Compare reference price to volume-weighted average fill, or
- Compare reference price to each individual fill component.
Without a defined rule, two people can compute different “slippage” from the same underlying order history.
Material limitation / failure mode
A common failure mode is using an unrelated benchmark for the reference. For instance, comparing a fill price to an earlier “typical” price, or to a historical spread average, can produce a number that is not actually the execution mismatch you care about. Slippage questions are answerable only when the reference and the fill definition are consistent.
Limitations and risks: uncertainty, what can vary, and how to verify
What can vary (and therefore what cannot be assumed)
Even if the definitions are stable, slippage-related outcomes vary with:
- Market conditions (liquidity and volatility affect how quickly quotes change).
- Execution timing and matching (how quickly orders reach matching and how fills are assigned).
- Costs beyond spread (fees, commissions, and other charges can alter net execution economics; they are not identical to slippage).
- Implementation differences (order types and execution models differ by provider/platform).
Because of these variables, historical relationships between spread behavior and execution quality do not establish future results.