The Greek letter "ει" (epsilon iota) is a diphthong in the Greek alphabet, representing a single vowel sound similar to the English long "e." It is commonly found in various Greek words and is pronounced as a single syllable. In modern Greek, "ει" is often used in everyday language and can appear in both formal and informal contexts. In ancient Greek, "ει" had a slightly different pronunciation and was used in various grammatical forms. It is also significant in the study of Greek linguistics and etymology, as it helps in understanding the evolution of the language over time.