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1. An impelling or restraining force; a compulsion.
2. Joint action.
3. 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.]