an active hostility or opposition, as between unfriendly or conflicting groups: the antagonism between the liberal and the conservative parties.
an opposing force, principle, or tendency: Her plan to become an actress met with the antagonism of her family.
Physiology. an opposing action, as by one muscle in relation to another.
Biochemistry, Pharmacology. the opposing action of substances, as drugs, that when taken together decrease the effectiveness of at least one of them (contrasted with synergism).
Ecology. a relationship between two species of organisms in which the individuals of each species adversely affect the other, as in competition. the inhibition of the growth of one type of organism by a different type that is competing for the same ecological niche.