Grammar. noting a construction of two or more words having the same syntactic function as an inflected word, as of Mr. Smith in the son of Mr. Smith, which is equivalent to Mr. Smith's in Mr. Smith's son.
British Dictionary definitions for un periphrastic
employing or involving periphrasis
expressed in two or more words rather than by an inflected form of one: used esp of a tense of a verb where the alternative element is an auxiliary verb. For example, He does go and He will go involve periphrastic tenses