al·pha / ˈalfə/ • n. 1. the first letter of the Greek alphabet (Α, α), transliterated as “a.” ∎ [as adj.] denoting the first of a series of items or categories, e.g., forms of a chemical compound: alpha interferon. ∎ short for alpha test. ∎ (Alpha) [followed by Latin genitive] the first (typically the brightest) star in a constellation: Alpha Centauri. ∎ [as adj.] relating to alpha decay or alpha particles: an alpha emitter. ∎ (of animals in a group) the socially dominant individual: he rose to be alpha male of his troop at the very early age of 16.2. a code word representing the letter A, used in radio communication.• symb. ∎ (α) a plane angle. ∎ (α) angular acceleration. ∎ (α) Astron. right ascension.