नस्ल (Breed) पालतू पशुओं का वर्गीकरण का एक सूचक है जिसमें उनके रूप-रंग, व्यवहार, अथवा किन्हीं अन्य लक्षणों के आधार पर, एक ही प्रजाति के जानवरों में भिन्न-भिन्न समूहों में वर्गीकृत किया जाता है। आम भाषा में कई बार यह शब्दावली पेड़-पौधों और फसलों के लिए भी प्रयुक्त होती है जबकि ऐसी फसलों को किस्म के नाम से बेहतर वर्गीकृत किया जाता है।

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. 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.