Definition
Execution is the completion of an order against available trading interest, producing a fill at a price, quantity, time, and venue.
In market context
An order can receive one fill, several partial fills, or no fill, depending on its instructions, market liquidity, and priority. Execution quality includes price, speed, likelihood, size, and total cost rather than only whether a trade occurred. The quote visible when an order is submitted can change before execution, particularly during volatility, so confirmations should be checked against the order instructions and resulting position.
Source
Use the primary source for fuller regulatory or market context.
