Market order
A market order is an instruction to buy or sell immediately at the best price currently available. The fill is guaranteed; the exact price is not.
A market buy fills at the ask, a market sell at the bid, so every market order pays the spread up front. And because the order takes whatever price is available when it arrives, the fill can differ from the quote you clicked — that difference is slippage. In liquid hours it is usually a fraction of a pip; in the seconds around a news release it can be many pips.
Market orders are the right tool when being in — or, more importantly, out — matters more than the last pip of price: momentum entries that will not wait, and exits. A triggered stop-loss becomes a market order for exactly this reason: when the level breaks, certainty of exit outranks precision. The patient alternative for entries is the limit order.
Example
EURUSD is quoted 1.0850 / 1.0851. A market buy of 1 standard lot fills at 1.0851, and the 1-pip spread costs $10 the moment it fills. Send the same order in the seconds after a rate decision and the fill might print 1.0854 instead — 3 pips of slippage, another $30. Being filled right now cost $40 before price moved at all; on 5 lots, $200.
In a prop-firm challenge
Slippage quietly widens planned risk: a 20-pip stop that fills 3 pips late risks 15% more than the plan says, which matters when trades are sized tightly against a daily loss limit. The reverse also holds — when a position must be closed to protect the limit, a market order is the only instruction that reliably gets you flat; a limit exit that never fills can let a loss run straight through a rule. Most challenge failures around news are execution stories, not analysis stories.