flat adverb
Definition - An adverb that has been derived from an adjective that does not change its form when it is used as an adverb (e.g. by adding -ly).

1. Some typical flat adverbs are: late, hard, fast, long, and high.

Example -

(1) The forward took a hard shot at the goalie.
(hard is used as an adjective)

(2) Messier shoots the puck hard.
(hard is used as an adverb)

Oxford English Dictionary -
The term's first citation is from 1871:
"The Flat Adverb is simply a substantive or an adjective placed in an adverbial position."
(J. Earle Philol. Eng. Tongue 361)

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