A hamadryad or hamadryas is a Greek mythological being that lives in trees. It is a particular type of dryad which, in turn, is a particular type of nymph. Hamadryads are born bonded to a certain tree on which its life depends. Some maintain that a hamadryad is the tree itself, with a normal dryad being simply the indwelling entity, or spirit, of the tree. If the tree should die, the hamadryad associated with it would die as well. For this reason, both dryads and the other gods would punish mortals who harmed trees.

A dryad is a tree nymph or tree spirit in Greek mythology. Drys (δρῦς) signifies "oak" in Greek. Dryads were originally considered the nymphs of oak trees specifically, but the term has evolved towards tree nymphs in general. Often their life force was connected to the tree in which they resided and they were usually found in sacred groves of the gods. They were considered to be very shy creatures except around the goddess Artemis, who was known to be a friend to most nymphs.