The term 'Alphabet Unit' refers to the three components that make up the English alphabet: a LETTER, an ATOM, and a VOWEL. A LETTER is a written symbol that represents a sound or combination of sounds, such as 'A' or 'B'. An ATOM is a unit of language consisting of a single sound, such as the 'm' in 'MAT'. A VOWEL is a speech sound produced with an open vocal tract, such as the 'a' in 'APPLE'. Together, these three components form the basis of the English alphabet.