A type of phrase that provides extra information - usually about time, place or manner - in a clause. A sentence or clause can contain several adverbials (which, unusually for an English syntax, can be located in various places). Adverbials are usually 'optional' elements in a clause - its central meaning being reasonably unaffected if they are left out.

Twice a day ADVERBIAL time
during the Summer ADVERBIAL time
furiously ADVERBIAL manner
in the gym ADVERBIAL place
on the rowing machine ADVERBIAL place

» Linguistic Library (Mike Green)