WebMeaning of A, a in English. A, a. noun. us / eɪ / uk / eɪ / plural As, A's or a's. A, a noun (LETTER) Add to word list. [ C or U ] the first letter of the English alphabet. SMART Vocabulary: related words and phrases. Letters of alphabets. ABCs. aitch. alphabetically. alphabetization. alphanumerically. G, g. H, h. I, i. iota. J, j. P, p. phi. pi.