Fauna is all of the animal life of any particular region or time. The corresponding term for plants is flora. Flora, fauna and other forms of life such as fungi are collectively referred to as biota.

Infauna are benthic organisms that live within the bottom substratum of a body of water, especially within the bottom-most oceanic sediments, rather than on its surface. Bacteria and microalgae may also live in the interstices of bottom sediments. In general infaunal animals become progressively smaller and less abundant with increasing water depth and distance from shore, whereas bacteria show more constancy in abundance, tending toward one billion cells per milliliter of interstitial seawater.

Avifauna: Birds (class Aves or clade Avialae) are feathered, winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. With around 10,000 living species, they are the most speciose class of tetrapod vertebrates.

In old-fashioned speech, critter and beast denoted a large domestic animal. The more restricted senses "a cow," "a horse," or "a mule" are still characteristic of the speech in specific regions of the United States. The use of critter among younger speakers almost always carries with it a jocular or informal connotation.