Any of several living arrangements between members of two different species, including mutualism, commensalism, and parasitism (qq.v.). Both positive (beneficial) and negative (unfavourable to harmful) associations are therefore included, and the members are called symbionts. The terms symbiosis and mutualism are sometimes equated and used interchangeably; this practice has resulted in some confusion. In a very broad sense any association between two species populations that live together is symbiotic, whether the species benefit, harm, or have no effect on one another.
Source: Encyclopaedia Britannica 2002