Definition
An order that typically becomes a market order when its stop price is reached; distinct from a stop-limit order.
Hypothetical example
A sell stop at 95 may fill below 95 if the market gaps from 100 to 90.
Keep in mind
The stop is not a guaranteed execution price. Trigger rules, hours and supported order types vary by broker and market.
Official reference
www.investor.gov ↗Examples are illustrative assumptions, not investment recommendations.