An alphabet is a standard set of letters (basic written symbols or graphemes) which is used to write one or more languages based on the general principle that the letters represent phonemes (basic significant sounds) of the spoken language. This is in contrast to other types of writing systems, such as logographies, in which each character represents a word, morpheme or semantic unit, and syllabaries, in which each character represents a syllable.

A true alphabet has letters for the vowels of a language as well as the consonants. The first true alphabet is believed to be the Greek alphabet, which is a modified form of the Phoenician alphabet. In other types of alphabet either the vowels are not indicated at all, as was the case in the Phoenician alphabet (such systems are known as abjads), or else the vowels are shown by diacritics or modification of consonants, as in the devanagari used in India and Nepal (these systems are known as abugidas or alphasyllabaries).

The English word alphabet came into Middle English from the Late Latin word alphabetum, which in turn originated in the Ancient Greek ἀλφάβητος (alphabētos), from alpha and beta, the first two letters of the Greek alphabet. Alpha and beta in turn came from the first two letters of the Phoenician alphabet, and originally meant ox and house respectively.