ecology 1. a major unit of vegetation comprising all plant communities that resemble each other in appearance and in major features of the environment, e.g., tropical rainforest, reed swamp —see association 2. a level in a classification based on ecological groupings of vegetation units that have broadly defined environmental and physiognomic factors in common —note this level is subject to revision as vegetation alliances and community associations are organized under the upper levels of the hierarchy This definition last updated 10/24/2008