• MUTUALISM (noun). The noun MUTUALISM has 1 sense: 1. the relation between two different species of organisms that are interdependent; each gains benefits from the other. Familiarity information: MUTUALISM used as a noun is very rare. MUTUALISM (noun). The noun MUTUALISM has 1 sense: 1. the relation between two different species of organisms that are interdependent; each gains benefits from the other. Familiarity information: MUTUALISM used as a noun is very rare.