An alliance is an informal grouping used in biological taxonomy. The term "alliance" is not a taxonomic rank defined in any of the nomenclature codes. It is used for any group of species,^[1] genera^[2] or tribes to which authors wish to refer, that have at some time provisionally been considered to be closely related.

The term is often used for a group that authors are studying in further detail in order to refine the complex taxonomy. For example, a molecular phylogenetics study of the Aerides–Vanda Alliance (Orchidaceae: Epidendroideae) confirmed that the group is monophyletic, and clarified which species belong in each of the 14 genera.^[3] In other orchid groups, the various alliances that have been defined do not correspond well to clades.^[4]

Historically, some 19th century botanical authors used alliance to denote groups that would now be considered orders.^[5] This usage is now obsolete, and the ICN (Article 17.2) specifies that such taxa are treated as orders.^{[citation needed]}