vocalize v.

late 14c., "spoken, oral," from Old French vocal (13c.), from Latin vocalis "sounding, sonorous, speaking," as a noun, "a vowel," from vox (genitive vocis) "voice" (from PIE root *wekw- "to speak"). In reference to music (as opposed to instrumental), first recorded 1580s; meaning "outspoken" first attested 1871. Vocal cords is from 1872; see cord.

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.