Iotacism (Greek: ?, iotakismos) or itacism is the process of vowel shift by which a number of vowels and diphthongs converged towards the pronunciation ([i]) in post-classical Greek and Modern Greek. The term "iotacism" refers to the letter iota, the original sign for ([i]), with which these vowels came to merge. The alternative term itacism refers to the new pronunciation of the name of the letter eta as ['ita] after the change.
Ancient Greek had a broader range of vowels (see Ancient Greek phonology) than Modern Greek does. Eta (?) was a long open-mid front unrounded vowel /?:/, and upsilon (?) was a close front rounded vowel /y/. Over the course of time, both vowels came to be pronounced like the close front unrounded vowel iota (?) [i]. In addition, certain diphthongs merged to the same pronunciation. Specifically, Epsilon-iota () initially became /e:/ in classical Greek, before later raising to (?) while, later, omicron-iota () and upsilon-iota () merged with upsilon (?). As a result of eta and upsilon being affected by iotacism, so were the respective diphthongs.
Iotacism caused some words with originally distinct pronunciations to be pronounced similarly, sometimes the cause of differences between manuscript readings in the New Testament. For example, the upsilon of , ? hymeis, hym?n "ye, your" (second person plural in respectively nominative, genitive) and the eta of , ? h?meis, h?m?n "we, our" (first person plural in respectively nominative, genitive) could be easily confused if a lector were reading to copyists in a scriptorium. (In fact, Modern Greek had to develop a new second-person plural, , while the first-person plural's eta was opened to epsilon, , as a result of apparent attempts to prevent it sounding like the old second-person plural.) As an example of a relatively minor (almost insignificant) source of variant readings, some ancient manuscripts spelled words the way they sounded, such as the 4th-century Codex Sinaiticus, which sometimes substitutes a plain iota for the epsilon-iota digraph and sometimes does the reverse.
English-speaking textual critics use the word "itacism" to refer to the phenomenon and extend it loosely for all inconsistencies of spelling involving vowels.