A free-swimming aquatic creature. It starts life as a larva, which attatches itself to a rock or other solid substance and develops into a polyp; eventually the polyp grows into the familiar bell-shaped organism.
Most jellyfish have tentacles and a basic nervous system, though they lack more complicated organs, and they are allegedly composed of about 95% water. They usually catch their prey by brushing potential victims with their poison-spouting, long tentacles, which can be anywhere from inches to several feet long.
Slang: various sex acts.
Sometimes, during low tide, one sees jellyfish lining the shores.