| Definition -
An adverbial that is a necessary sentence element (i.e. if you remove it, the sentence ceases to be well-formed).
1. As opposed to adverbial adjuncts, so called because they can be removed from well-formed sentences and the sentences remain well-formed.
Example - Bob is in the garden
(If you remove the adverbial complement in the garden, the sentence ceases to be well-formed.)