Adjunct

Adjunct is a constituent in an adjoined position.
 * "Adjuncts (of place, time, manner, reason, etc....) are optional, or structurally dispensable, constituents of the sentence: they may be removed without affecting the remainder of the sentence." (Lyons 1968:334)

Examples
In [X Y [X ...]] Y is Chomsky -adjoined to X.

(i) John killed Bill in Central Park on Sunday.

(ii) John killed Bill in Central Park.

(iii) John killed Bill.

Comment
Usually the term 'adjunct' refers to base-generated adjuncts only.

Other languages
German Adjunktion Chinese 附加语

Link
Utrecht Lexicon of Linguistics