Calisthenics is the usual American spelling of callisthenics (British English often keeps the extra l). It means exercise you do with your own body: push-ups, sit-ups, squats, jumping jacks — strength and looseness without gym machines, and often without weights. The name was built from Greek for “beauty” and “strength,” so the old picture was graceful, strong movement, not only sweating. Though actually, American schools and coaches also use it for a short warm-up of simple moves, which can sound rather official or a little old-fashioned; many younger people just say bodyweight training, or name the move. In parks you may also hear it for harder bar work (pull-ups, muscle-ups). So the word stretches from gentle PE to serious street workouts. This one-l spelling is the one American books and apps will show you.