Iotacism (Greek: ?, iotakismos) is the process by which a number of vowels and diphthongs in Ancient Greek converged in pronunciation so they all now sound like iota ([i]) in Modern Greek. In the case of the letter eta specifically, the process is known as itacism (from the resulting pronunciation of the letter's name as ['ita]).
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 "you, your" (second person plural in respectively NOM, GEN) and the eta of , ? h?meis, h?m?n "we, our" (first person plural in respectively NOM, GEN) 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 fronted 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.