An acronym for knock-out, knock-in; a combination of barrier events (in relation to a barrier option, specifically, a double barrier option) that takes place in the following sequence: 1) a knock-out event and 2) a knock-in event. If the underlying price or rate crosses the knock-out barrier before it crosses the knock-in barrier, then the option ceases to exist (i.e., terminates).

This situation is known as knock-out dominance: the option can terminate (knock out) at any time, even including after it knocks in. If the option knocks in, it knocks into a knock-out option (KO option).