Referring to something which does not go together with something else.
Referring to drugs which must not be used together because they undergo chemical change and the therapeutic effect is lost or changed to something undesirable.
Referring to tissue which is genetically different from other tissue, making it impossible to transplant into that tissue.
Typically, this refers to materials that can’t be combined or used together due to negative interactions or conflicting properties. Pharmacists maintain a stringent list of such incompatible substances that are never mixed in the same medication.