Heavy NP shift is an operation that involves re-ordering (shifting) a "heavy" noun phrase (NP) to a position to the right of its canonical position under certain circumstances. The heaviness of the NP is determined by its grammatical complexity; whether or not shifting occurs can impact the grammaticality of the sentence.
Ross (1967) is thought to have been the first to systematically investigate the properties of heavy NP shift. Although the term heavy NP shift derives from the theoretical framework of transformational grammar, which describes the process in terms of movement of the NP, linguists working in other frameworks also use this term. And in approaches to syntax where NP is analyzed as a determiner phrase (DP), heavy NP shift is called heavy DP shift.