A Snake is a scaly, limbless, elongate reptile from the order Squamata. An old synonym for snake is serpent (which comes from Old French, and ultimately from *serp-, "to creep");; in modern usage this usually refers to a mythic or symbolic snake.

The phylogeny of snakes is poorly known due to the fact that snake skeletons are typically small and fragile, making fossilization unlikely. It has however been generally agreed, on the basis of morphology, that snakes descended from lizard-like ancestors. Recent research based on genetics and biochemistry confirms this; snakes form a venom clade with several extant lizard families.

Recent fossil evidence suggests that snakes directly evolved from burrowing lizards, either varanids or some other group. An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, fully terrestrial. One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo, although it also is semi-aquatic. As these ancestors became more subterranean, they lost their limbs and became more streamlined for burrowing. Features such as the transparent, fused eyelids and loss of external ears, according to this hypothesis, evolved to combat subterranean conditions (scratched corneas, dirt in the ears);. According to this hypothesis, snakes re-emerged onto the surface of the land much as they are today.