Definition

Classification is sorting living things into groups based on how they are alike, with big groups like animals split into smaller ones like mammals and on down to a single species, so a house cat is an animal, a vertebrate, a mammal, a carnivore, and finally the species Felis catus, a system that works like folders inside folders so scientists everywhere know exactly which organism they mean. Formally, classification (taxonomy) organizes organisms into a ranked hierarchy from domain, kingdom, phylum, class, order, family, genus, down to species, using Linnaeus's two-part scientific naming (binomial nomenclature), and modern classification reflects ancestry rather than mere appearance, which is why whales are grouped with mammals rather than fish. At the research level, classification aims to reflect phylogeny, so named groups should be monophyletic clades containing an ancestor and all its descendants, reconstructed from morphological and molecular characters using cladistic methods; the three-domain system (Bacteria, Archaea, Eukarya) that Carl Woese proposed from ribosomal RNA sequences replaced the older five-kingdom scheme, and molecular data have reorganized many traditional groups, such as placing birds within reptiles and fungi closer to animals than to plants, leaving Linnaean ranks useful for convenience but without a fixed biological meaning across lineages.