emphasize v.

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, in context, more than would ordinarily be meant by the words. Hence "the mode of delivery appropriate to suggestive expression, rhetorical stress," and thence, in general, extra stress or force of voice given to the utterance of a word, phrase, or part of a word in speech as a clue that it implies something more than literal meaning. In pure Latin, significatio.

word-forming element used to make verbs, Middle English -isen, from Old French -iser/-izer, from Late Latin -izare, from Greek -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached.

The variation of -ize and -ise began in Old French and Middle English, perhaps aided by a few words (such as surprise, see below) where the ending is French or Latin, not Greek. With the classical revival, English partially reverted to the correct Greek -z- spelling from late 16c. But the 1694 edition of the authoritative French Academy dictionary standardized the spellings as -s-, which influenced English.

In Britain, despite the opposition to it (at least formerly) of OED, Encyclopaedia Britannica, the Times of London, and Fowler, -ise remains dominant. Fowler thinks this is to avoid the difficulty of remembering the short list of common words not from Greek which must be spelled with an -s- (such as advertise, devise, surprise). American English has always favored -ize. The spelling variation involves about 200 English verbs.