Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdomAnimalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also heterotrophs, meaning they must ingest other organisms for sustenance.

Most known animal phyla appeared in the fossil record as marine species during theCambrian explosion, about 542 million years ago.

The word "animal" comes from the Latin word animalis (meaning with soul, from anima, soul). In everyday colloquial usage, the word usually refers to non-human animals.^[1] Frequently, only closer relatives of humans such as mammals and other vertebrates are meant in colloquial use.^[2] The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects and humans.^[3]

Animals have several characteristics that set them apart from other living things. Animals areeukaryotic and mostly multicellular, which separates them from bacteria and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae.^[6] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.All animals are motile, if only at certain life stages. In most animals, embryos pass through a blastula stage, which is a characteristic exclusive to animals.