A, or a, is the first letter and the first vowel of the modern English alphabet and the ISO basic Latin alphabet. Its name in English is a (pronounced ), plural aes. It is similar in shape to the Ancient Greek letter alpha, from which it derives. The uppercase version consists of the two slanting sides of a triangle, crossed in the middle by a horizontal bar. The lowercase version can be written in two forms: the double-storey a and single-storey ɑ. The latter is commonly used in handwriting and fonts based on it, especially fonts intended to be read by children, and is also found in italic type.

In the English grammar, "a", and its variant "an", are indefinite articles.