Definition - A word or a group of words that functions like an adverb, but which isn't an adverb.

1. Adverbials answer the questions Where?, When?, Why? and How?

Example -

In the following, the bolded words are adverbials:
(1) Arturo answered immediately. (adverb)
(2) Arturo answered in Italian. (prepositional phrase)
(3) Arturo answered last night. (noun phrase)
(4) Arturo answered in Italian because he had an urge to do so. (adverbial clause)

Etymology -
The word derives from the Latin adverbium, that which is added to a verb (from Latin ad, to + verbum, word).
Note: The Latin term was invented by Flavius Sosipater Charisius who coined it to translate the Greek word epirrhema, adverb (from Greek epi, upon + rhema, verb).

Oxford English Dictionary -
Its first citation in this sense is from 1924:
"The term Adverbials is a convenient designation of adverbs and their equivalents, viz. Group-Adverbs, Adverbial Phrases and Adverbial Clauses."
(H. E. Palmer Gram. Spoken Eng. ii. 234)

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