Syncretism is the technical term for what we might think of as “mixing up on purpose” or “mashing up” — combining disparate elements. Like items from popular culture (Casper the Friendly Ghost, the Three Stooges) introduced into Shakespeare.
attempted union or reconciliation of diverse or opposite tenets or practices, esp. in philosophy or religion [cites from 1618 on]
– in the 20th century, expanded to merging elements of culture (like the Virgin Mary with the Earth Mother) and more generally to combinations in art, literature, music, etc.
The natural extension of the term to linguistics would have been to “blends” of various kinds: of words (in portmanteaus, in particular) and of syntactic constructions, and even to language “mixtures” of various kinds. But these phenomena have their own terminology, and syncretism has come to be used instead for:
The merging of two or more inflectional categories. [first OED cite in WNI1 in 1909; prominent in Bloomfield's Language (1933)]
In cultural syncretism, the contributions of the combining elements are still to some extent discernible, but in morphological syncretism, we have entities that are identical in form and can be distinguished only by their content. So: English has a pervasive and systematic identity between the PST and the PSP, which are identical for all “regular” verbs (for jump, PST jumped, PSP jumped) and for many others as well (for think, PST thought, PSP thought); this is syncretism on a grand scale. Nevertheless, the categories are distinct in form for a number of verbs (for sing, PST sang, PSP sung; for give, PST gave, PSP given; etc.), which is why we posit separate categories in the first place.