1. (in biology) Describing a character that is shared by a group of species because it is inherited from a common ancestor. Such characters, called homologies, are used in cladistics to determine the evolutionary relationships of species or higher taxa. They are divided into two types: a shared derived homology (see synapomorphy) is unique to a particular group and may be used to define a monophyletic group; a shared ancestral homology (see plesiomorphy) is not unique to the group, or may not be exhibited by all descendants of the ancestor in which it arose (see paraphyletic). Even though homologous features share the same evolutionary origin, they may have developed different functions. For example the wings of a bat, the flippers of a dolphin, and the arms of a human are homologous organs, having evolved from the paired pectoral fins of a fish ancestor. Compare analogous.

2. (in molecular biology) Describing sequences of nucleotides (or amino acids) at corresponding sites of different nucleic acids (or proteins) that show similarity because the molecules are descended from a common ancestral molecule (see sequence analysis). The term is sometimes used more loosely, but incorrectly, to describe sequences that are merely similar, when no evolutionary relationship is implied or can be established. See orthologous; paralogous. See also conserved sequence.