Stop order
A stop order becomes a market order when price reaches your level: a buy above the current price, a sell below it.
A stop order is used to enter in the direction of a move: buy if price breaks resistance, sell if it drops below support. On MetaTrader these are Buy Stop and Sell Stop orders.
It is also how a stop loss works: once the level is hit, the position is closed at market. Like any market order, it can suffer slippage, especially in a gap.
There is a variant, the stop limit order, which turns into a limit order instead of a market order. It protects against slippage, but may not be filled if price jumps past your level.