A breed is a specific group of domestic animals having homogeneous appearance (phenotype), homogeneous behavior, and/or other characteristics that distinguish it from other organisms of the same species. In literature, there exist several slightly deviating definitions.Breeds are formed through genetic isolation and either natural adaptation to the environment or selective breeding, or a combination of the two. Despite the centrality of the idea of "breeds" to animal husbandry and agriculture, no single, scientifically accepted definition of the term exists. It was shown by set-theoretic means that for the term breed an infinite number of different definitions, which more or less meet the common requirements found in literature, can be given. A breed is therefore not an objective or biologically verifiable classification but is instead a term of art amongst groups of breeders who share a consensus around what qualities make some members of a given species members of a nameable subset.

Another point of view is that a breed is consistent enough in type to be logically grouped together and when mated within the group produce the same type.When bred together, individuals of the same breed pass on these predictable traits to their offspring, and this ability – known as "breeding true" – is a requirement for a breed. Plant breeds are more commonly known as cultivars. The offspring produced as a result of breeding animals of one breed with other animals of another breed are known as crossbreeds or mixed breeds. Crosses between animal or plant variants above the level of breed/cultivar (i.e. between species, subspecies, botanical variety, even different genera) are referred to as hybrids