Extraposition

Extraposition is originally a movement rule that moves CPs and PPs to the right periphery of the sentence.

Example
in (ii) the relative clause which I like is extracted from the subject NP and moved to the right. PPs can be extraposed as well: in (iv) the PP about the Sovjet Union is extraposed.

(i)   [NP Many paintings of young artists [CP which I like]] are on sale (ii)  [NP Many paintings of young artists t] are on sale [CP which I like] (iii) [NP Many books [PP about the Sovjet Union]] will appear soon (iv)  [NP Many books t ] will appear soon [PP about the Sovjet Union]

In general, extraposition is optional and clause bound. In Dutch, complement clauses undergo extraposition obligatorily, as shown by the contrast between (v) and (vi).

(v)  * Kees heeft [CP de prijs  aan te nemen] geweigerd Kees has      the prize prt to accept refused' (vi)   Kees heeft t geweigerd [CP de  prijs aan te nemen] Kees has    refused      the prize prt to accept 'Kees has refused to accept the prize'

Finite complement clauses have to be extraposed as well:

(vii)  Kees heeft t besloten [CP dat hij de prijs zal weigeren] 'Kees has    decided     that he  the prize will refuse'

Often the term 'extraposition' merely refers to the state of being in a right peripheral position.

Link
Utrecht Lexicon of Linguistics

Other languages
German Extraposition (de)