Understanding and preserving intra-specific diversity (ISD) is important for species conservation. However, ISD units do not have taxonomic standards and are not universally recognized. The terminology used to describe ISD is varied and often used ambiguously. We compared classical and authoritative definitions of terms used to describe ISD with terms used in recent studies of three fish taxa: sticklebacks (Gasterosteidae), Pacific salmon and trout (Oncorhynchus spp., “PST”), and lampreys (Petromyzontiformes). Our review revealed the terminological ambiguity of “races” and “subspecies”, found similar definitions of “subspecies” and “ecotype”, and of “ecotype” and “reaction norms”. “Species pairs” describes two phenotypes; however, in some situations more than one phenotype may occur. “Ecotype” was originally used to describe patterns in genes and ecology, and recent studies employing this term tend to report a genetic basis in ISD. Ecotype is used most frequently in genetic- and evolution-based journals. By contrast, “life history” includes biological parameters that affect population growth and decline, and this term tends to be used in organismal- and ecology-based journals. When the genetic or demographic components of ISD are not well understood, a conservative approach would be to refer to expressions of this diversity as “phenotypes”. The nature of human interests in particular taxa could influence how these organisms are studied, and hence the ways in which their ISD is understood, described, and conserved.