(medicine, chemistry, chiefly in the plural) An incompatible substance; one of a group of things that cannot be placed or used together because of a change of chemical composition or opposing medicinal qualities.

(philosophy) A consequent of a contrary.

Of two things: that cannot coexist; not congruous because of differences; unable to function together due to dissimilarities.

(chemistry) Incapable of being together without mutual reaction or decomposition, as certain medicines.