incompatible

Short definition from Wiktionary

Adjective

1. 1
   Of two things: that cannot coexist; not congruous because of differences; unable to function together due to dissimilarities.
2. 2
   Incapable of being together without mutual reaction or decomposition, as certain medicines.

Noun

1. 1
   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.
2. 2
   A consequent of a contrary.