Stop-loss order
An order to sell, or buy to cover, once a price reaches a set level, to limit losses.
A stop-loss order becomes a market order when the price trades through a chosen trigger. It is meant to cap the loss on a position without constant monitoring. The trigger does not guarantee the fill price: in a fast market the order can execute well beyond the trigger.
On our site, read a stop level as a reference point for risk, not as a guarantee of the exit price.
Traders care because stops can be triggered by brief spikes and cluster around round numbers. Gaps, halts and thin liquidity make the realized exit price worse than the trigger.
Worked example (illustrative numbers, not real data): a long bought at 100 has a stop-loss at 95. A quick drop to 94 at the open fills the order at 93.80, a loss of 6.20 per share rather than the 5.00 the trigger implied.