syncretism

The ancient Greeks mainly used the term synkrētismos to describe the joining together of Greeks in opposition to a common enemy. In the early 17th century, English speakers adopted the term in the anglicized form syncretism to refer to the union of different religious beliefs. Three centuries later, lexicographers of the 1909 edition of Webster’s New International Dictionary of the English Language added a new definition of syncretism ("the union or fusion into one or two or more originally different inflectional forms, as of two cases"), but this specialized sense is rarely encountered outside of the field of linguistics. Some related terms that you might encounter are syncretize ("to attempt to unite and harmonize"), syncretist ("one who advocates syncretism"), and syncretic and syncretistic ("characterized or brought about by syncretism").