Syncretism is a union or attempted fusion of different religions, cultures, or philosophies — like Halloween, which has both Christian and pagan roots, or the combination of Aristotelian philosophy with the belief system of the early punk rock practitioners.
Syncretism may apply to something whose parts sync well, like salsa dance (a merging combination of African, Latin American, and European cultures), or something whose parts merge less successfully. Syncretism is also a word used for merging different inflection word forms, so that a word can serve more than one function. Examples in English are you, which can be an object or a subject, or pet, which is the same in the past and present tense.
the union (or attempted fusion) of different systems of thought or belief (especially in religion or philosophy)
the fusion of originally different inflected forms (resulting in a reduction in the use of inflections)