In biological classification, class (Latin: classis) is: * a taxonomic rank. Other well-known ranks in descending order of size are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the prefix sub-: subclass (Latin: subclassis). * a taxonomic unit, a taxon, in that rank. In that case the plural is classes (Latin classes)