A knife with a blade that folds closed is called a jackknife. Most jackknives are compact enough to fit in a pocket, but we don't suggest your taking one to school.

Another name for a jackknife is a pocketknife. Some jackknives have several different blades, all of which fold tidily (and safely) into the body of the knife, while others unfold to reveal a single blade. The bend of a jackknife inspired the name of the dive also called the jackknife, in which the diver's body folds into a V shape and then straightens out before the diver hits the water.

Definitions of jackknife
  1. noun
    a large knife with one or more folding blades
    synonyms: clasp knife
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    type of:
    pocket knife, pocketknife
    a knife with a blade that folds into the handle; suitable for carrying in the pocket
  2. noun
    a dive in which the diver bends to touch the ankles before straightening out
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    type of:
    dive, diving
    a headlong plunge into water
  3. verb
    dive into the water bending the body at the waist at a right angle, like a jackknife
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    type of:
    plunge into water
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