1 In the Zurich–Montpellier school of phytosociology, the basic unit of vegetation, an abstract entity that is defined floristically from field data or relevés. Each association has a distinctive faithful species and a group of constant (i.e. high-presence) species which give it a coherent structure. The companion species of the association often form the faithful species of the next and succeeding hierarchical levels, alliances, and orders into which similar associations are grouped.
2 In the British and American phytosociological traditions, a community that is united physiognomically as well as floristically; commonly it is a climax community in which the species dominants are those of the upper vegetation layer. There are usually several co-dominant species; if there is only one dominant the community is called a ‘consociation’. See also formation.