In scientific usage, a multicellular organism that is usually mobile, whose cells are not encased in a rigid cell wall (distinguishing it from plants and fungi) and which derives energy solely from the consumption of other organisms (distinguishing it from plants).
“A cat is an animal, not a plant. Humans are also animals, under the scientific definition, as we are not plants.”
In non-scientific usage, any member of the kingdom Animalia other than a human.