1. A tube designed for breathing while swimming with one's face in the water. Usually a snorkel is worn with a diving mask. A snorkel has a mouthpiece on one end, which is held in the mouth gripped by the teeth. The tube close to the mouthpiece is curved over to one side of the face, then a straight section sticks out towards the back of the head so that that its open end will be out of the water when the wearer is floating on his or her stomach. This allows the wearer to breathe freely without having to raise the head out of the water.
  2. Also, a similar (but bigger!) tube used by submarines to take in fresh air and give off exhaust. This lets a sub stay underwater for longer than it otherwise could.