(1):

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

(2):

(n.) An incompatible substance; esp., in pl., things which can not be placed or used together because of a change of chemical composition or of opposing medicinal qualities; as, the incompatibles of iron.

(3):

(a.) Not compatible; so differing as to be incapable of harmonious combination or coexistence; inconsistent in thought or being; irreconcilably disagreeing; as, persons of incompatible tempers; incompatible colors, desires, ambition.