A price you decide in advance to sell at if a holding falls to it, so the decision is made before the loss is emotional rather than during it. The trade-off is real: set it tight and ordinary volatility takes you out of a position that then recovers, set it loose and it barely limits anything. It works better on a stock you bought for a price move than on one you bought to hold for years. PSEye lets you record a stop level per holding and flags it when the price crosses.
See every term in the full PSE investing glossary.