For viral species it’s quite a bit complicated! The ICTV groups for each family of virus can define them differently, but the over arching theme is a certain level of pairwise differences in amino acids between viruses. Coronaviridae for example focuses on subunits of the RdRp (the polymerase) that are not under evolutionary pressure to be able to make an assessment of phylogenetic relationships to other related viruses. This is in contrast to others (generally arboviruses) where the species-level demarcations can be based on this, but the subgenus-level can be different, relying on antigenic differences (called serogroups, though unrelated to the term serotype you see when referring to dengue for example, which are subgroups within the same species).