A herd refers to a social grouping of certain animals of the same species, either wild or domestic, and also to the form of collective animal behavior associated with this (referred to as herding) or as a verb, to herd, to its control by another species such as humans or dogs.

The term herd is generally applied to mammals, and most particularly to the grazing ungulates that classically display this behaviour. Different terms are used for similar groupings in other species; in the case of birds, for example, the word is flocking, but flock may also be used, in certain instances, for mammals, particularly sheep or goats. A group of quail is often referred to as a covey. Large groups of carnivores are usually called packs, and in nature a herd is classically subject to predation from pack hunters.

Special collective nouns may be used for particular taxa (for example a flock of geese, if not in flight, is sometimes called a gaggle) but for theoretical discussions of behavioural ecology, the generic term herd can be used for all such kinds of assemblage.^{[citation needed]}