emphasis (n.) Look up emphasis at Dictionary.com
1570s, "intensity of expression," from Latin emphasis, from Greek emphasis "an appearing in, outward appearance;" in rhetoric, "significance, indirect meaning," from emphainein "to present, exhibit, display, let (a thing) be seen; be reflected (in a mirror), become visible," from assimilated form of en "in" (see en- (2)) + phainein "to show" (from PIE root *bha- (1) "to shine").

In Greek and Latin, originally a figure of expression implying more than would ordinarily be meant by the words, it developed a sense of "extra stress" given to a word or phrase in speech as a clue that it implies something more than literal meaning. In pure Latin, significatio.