| 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.
(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)