In community ecology and phytosociology an association is a type of ecological community with a predictable species composition, consistent physiognomy (structural appearance) which occurs in a particular habitat type. The term was first coined by Alexander von Humboldt and formalised by the International Botanical Congress in 1910.
An association can be viewed as a real, integrated entity shaped either by species interactions or by similar habitat requirements, or it can be viewed as merely a common point along a continuum. The former viewed was championed by American ecologist Frederic Clements, who viewed the association as a whole that was more than the sum of its parts, and by Josias Braun-Blanquet, a Swiss-born phytosociologist. On the other end of the argument was American ecologist Henry Gleason, who saw these groupings of plant species as a coincidence produced by the "fluctuation and fortuitous immigration of plants, and an equally fluctuating and variable environment".