al·pha (ălʹfə)n. 1. The first letter of the Greek alphabet. See table at alphabet. 2. The first one; the beginning. 3.Chemistry. The first position from a designated carbon atom in an organic molecule at which an atom or radical may be substituted. 4.Astronomy. The brightest or main star in a constellation. 5. The mathematical estimate of the return on a security when the return on the market as a whole is zero. Alpha is derived from a in the formula Ri = a + bRm, which measures the return on a security (Ri) for a given return on the market (Rm) where b is beta.adj. 1. Being the highest ranked or most dominant individual of one's sex. Used of social animals: the alpha female of the wolf pack. 2.Chemistry. Closest to the functional group of atoms in an organic molecule. 3. Alphabetical. [Greek, of Phoenician origin. See ʾlp in Semitic Roots.]