Entry 2 senses · 2 variants Webster, 1913 Ac · IPA /ˈeɪsˈiː/ n. n. 01 n. the chemical symbol for actinum, a radioactive element. 1. the chemical symbol for actinum, a radioactive element. Also: actinum Syn. actinium 02 n. an electric current that reverses direction sinusoidally. Alternative to direct curent, DC. 1. an electric current that reverses direction sinusoidally. Alternative to direct curent, DC.[acron.] Syn. alternating current