Definition of "adverbial"

And in (123) below, a (bracketed) Adverbial Phrase has undergone WH MOVEMENT: (123) (a) [How quickly] will he drink the beer —? (123) (b) [How carefully] did he plan his campaign —? (123) (c) [How well] did he treat her —?

1988, Andrew Radford, chapter 9, in Transformational grammar: a first course, Cambridge, UK: Cambridge University Press, page 495