| Definition -
An adverb that modifies either a whole sentence or an independent clause.
1. It conveys the speaker's overall mood, sentiments, or attitude. It does not modify a particular verb, adjective, or adverb within a sentence.
(1) Hopefully, he'll be back by midnight.
(Translation: I hope he'll be back by midnight.)
(2) Fortunately, he had already decided to kiss her.
Oxford English Dictionary - The term's first citation is from 1892:
"*Sentence-adverbs. The answer to the question is he here? can be either the affirmative yes or the negative no. It is evident that yes and no are sentence-modifying adverbs and at the same time sentence-words like come! John!, alas!"
(H. Sweet New Eng. Gram. I. 127)