Ecology Any of the reciprocal actions or effects, such as symbiosis, that can occur in a community.
[Middle English coaccioun, from Latin coāctiō, coāctiōn-, a collecting, from coāctus, past participle of cōgere, to collect, condense; see coagulum. Senses 2 and 3 : co- + action.] co·ac'tive adj., co·ac'tive·ly adv.