Iotacism (Greek: ιωτακισμός, 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 [ˈita]).
Ancient Greek had a broader range of vowels (see Ancient Greek phonology) than Modern Greek does. Eta (η) was a long open-mid front unrounded vowel /ɛː/, and upsilon (υ) 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 (ι) [i]. In addition, certain diphthongs merged to the same pronunciation. Specifically, Epsilon-iota (ει) initially became /eː/ in classical Greek, before later raising to (ι) while, later, omicron-iota (οι) and upsilon-iota (υι) merged with upsilon (υ). 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 ὑμεῖς, ὑμῶν hymeis, hymōn "you, your" (second person plural in respectively NOM, GEN) and the eta of ἡμεῖς, ἡμῶν hēmeis, hēmōn "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, εσείς, while the first-person plural's eta was fronted to epsilon, εμείς, 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.