co·ac·tion 
(kō-ăkshən)
(kō-ăkshən)n.
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.]
co·active adj.
co·active·ly adv.