A knife with a blade that folds into the handle; suitable for carrying in the pocket WordNet 3.0 & Open English WordNet

A pocketknife is a knife with one or more blades that fold into the handle. Wikipedia

Knife that can be carried in a pocket Wikidata

A knife with blades or tools that the user can fold or retract into its handle, and of a size small enough for carrying safely and handily in a pocket. Since the late 19th century the term "penknife" has not been distinct from "pocketknife", but the latter tends to refer to larger and more robust versions, sometimes with more attached tools, suited to heavier duty for casual or ad hoc applications outdoors or in workshops. Wiktionary

Small knife whose blades or tools can fold in its handle. Wiktionary (translation)