Dryads (or "tree spirits") are nymphs associated with Greek mythology and Hinduism, which live near, or in, trees. Dryads are born bonded to a specific tree, originally, in the Indo-European Celtic-Druidic culture, an oak tree. Drys in Greek signifies 'oak,' from an Indo-European root *derew(o)- 'tree' or 'wood.' In the primitive times, the Greeks imagined, people were able to live on acorns. Caryatids were associated with various edible nut trees, before they were pressed into architectural uses. And the dryads of ash trees were called the Meliai. The ash-tree sisterhood tended the infant Zeus in Rhea's Cretan cave. Rhea had born the Meliai herself, made fertile by the cast-away genitals of Ouranos.
If the nymphs lived in the trees, they were referred to as hamadryads, like Atlantia one of the wives of Danaus; otherwise they were simply dryads. Dryads, like all nymphs were supernaturally long-lived, but if the tree died, the dryad associated with it died as well. For that reason, dryads and the Greek gods punished any mortals who harmed trees without first propitiating the tree-nymphs.
See also the myth of Daphne, who was pursued by Apollo and became a dryad associated with the laurel.