A fish is a cold-blooded aquatic vertebrate with scales and gills, like a shark, a trout, or a pike. To fish is to try to catch a fish.
Fish comes from an old German version of the Latin word for "fish," piscus, from which you might recognize the astrological sign Pisces. The word can describe either the creature or the meat from the creature. When fish is a verb, it can be used literally for the act of trying to catch a fish, or metaphorically for trying to find something, like fishing for someone's name in your memory.