Core terms
Slippage
The difference between the price you expected, against a stated reference, and the average price you actually received.
The reference has to be named: the quote when the order was sent (arrival price), the price at which a signal appeared, or the stop price. Slippage on a stop is common in thin books: the stop triggers at its price but fills at the next available price. In an Illustrative example, a stop at $2.81 that fills at $2.79 adds two cents per share and turns a planned −1R loss into about −1.31R after fees.
Slippage tends to grow with order size relative to displayed liquidity, with spread width and with speed, and it can be large around halts, when a stop cannot execute until trading resumes.
What traders should verify
Log the reference price and the fill for every order; compute slippage per share and in R. In simulators, add a slippage penalty because simulated fills are usually optimistic.
Do not conflate
Trader usage; there is no single official definition, and tools measure it against different references. A stop price is a trigger, not a fill price.