flick knife

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a pocketknife with a blade that springs open at the press of a button