The chapter discusses the concept of species and the process of speciation, emphasizing the need for a unified and terminologically consistent approach to understanding these concepts. It reviews various species definitions, including the Biological Species Concept, Isolation Concept, Recognition Concept, Evolutionary Species Concept, Ecological Species Concept, Cohesion Species Concept, Phylogenetic Species Concept, Genealogical Species Concept, Phenetic Species Concept, and Genotypic Cluster Definition. Despite the diversity of these definitions, they all ultimately describe variations of the general lineage concept of species, which equates species with segments of population-level evolutionary lineages. The author argues that all modern species definitions are based on the same underlying concept, but differ in their criteria for identifying or delimiting species. These criteria are related to the events and subprocesses that occur during the process of speciation, such as initial separation, cohesion, interbreeding, and divergence. The chapter proposes a revised terminology to distinguish between species concepts (general ideas about the kind of entity designated by the term "species") and species criteria (standards for judging whether an entity qualifies as a species). The revised terminology aims to provide a more informative and accurate description of the diverse ideas about species, unifying them under a single conceptual framework. This framework helps to clarify the relationship between species concepts and criteria, and to address the potential for misinterpretation and confusion in the field of evolutionary biology.The chapter discusses the concept of species and the process of speciation, emphasizing the need for a unified and terminologically consistent approach to understanding these concepts. It reviews various species definitions, including the Biological Species Concept, Isolation Concept, Recognition Concept, Evolutionary Species Concept, Ecological Species Concept, Cohesion Species Concept, Phylogenetic Species Concept, Genealogical Species Concept, Phenetic Species Concept, and Genotypic Cluster Definition. Despite the diversity of these definitions, they all ultimately describe variations of the general lineage concept of species, which equates species with segments of population-level evolutionary lineages. The author argues that all modern species definitions are based on the same underlying concept, but differ in their criteria for identifying or delimiting species. These criteria are related to the events and subprocesses that occur during the process of speciation, such as initial separation, cohesion, interbreeding, and divergence. The chapter proposes a revised terminology to distinguish between species concepts (general ideas about the kind of entity designated by the term "species") and species criteria (standards for judging whether an entity qualifies as a species). The revised terminology aims to provide a more informative and accurate description of the diverse ideas about species, unifying them under a single conceptual framework. This framework helps to clarify the relationship between species concepts and criteria, and to address the potential for misinterpretation and confusion in the field of evolutionary biology.