Source : Webster's Revised Unabridged Dictionary (1913)

Adverbial \Ad*ver"bi*al\, a. [L. adverbialis: cf. F. adverbial.]
   Of or pertaining to an adverb; of the nature of an adverb;
   as, an adverbial phrase or form.

Source : WordNet®

adverbial
     adj : of or relating to or functioning as an adverb; "adverbial
           syntax"

adverbial
     n : a word or group of words function as an adverb