Chondrichthian is a term that refers to cartilaginous fish, which include sharks, rays, and skates. These creatures are characterized by their unique skeletal structure and have been around for over 400 million years. There are a number of synonyms that can be used to describe chondrichthians, including elasmobranchs, selachians, and cartilaginous fish. Elasmobranchs are a specific type of chondrichthian that includes sharks, skates, and rays, while selachians are a type of chondrichthian that only refers to sharks. Cartilaginous fish is a broader term that encompasses all chondrichthians, including sharks, rays, skates, and chimaeras.