A Jumping Jack, or side straddle hop as it is called in the United States military (also commonly known as a star jump, especially to children of Commonwealth nations), is a physical jumping exercise performed by jumping to a position with the legs spread wide and the hands touching overhead and then returning to a position with the feet together and the arms at the sides. The name comes from the traditional toy of the same name and is also known as a jack in the box.
More intensive versions of this jump include bending down (over) and touching the floor in between each jump.
This exercise is not, however, suitable for increasing jumping endurance.
A similar jump exercise, as coined by actor Jason David Frank, is called half-jacks. They are just like regular jumping jacks, but the arms go halfway above the head instead of all the way above the head. The arms also hit the sides to help tighten the jump.
Although he played no part in inventing the exercise, jumping expert Jack LaLanne takes credit for having popularized it in the United States as part of the jumping exercise routine he promoted on decades of television programs.