flick knife

Wiktionary

1. n. UK A type of knife which has the blade mounted on a pivot so that it can fold back sideways into the handle when not in use, but can flick out straight, usually with a spring-loaded mechanism, when required. Such a knife is almost always used as a weapon.

WordNet 3.0

1. n. a pocketknife with a blade that springs open at the press of a button