bisextus

A New Latin Dictionary by Charlton T. Lewis Ph.D. and Charles Short, LL. D.

bĭ-sextus (erroneously bissextus; v. bis fin.), i, m., sc. dies (bĭsextum, i, n., Censor. 20; Amm. 26, 1, 7) [bis],

an intercalary day; so called, since the 24th of February = VI. Cal. Mart., was doubled: bisextus est post annos quattuor unus dies adjectus, Isid. Orig. 6, 17, 25; Dig. 50, 16, 98; 4, 4, 3, § 3; Macr. S. 1, 14; Aug. Trin. 4; Isid. Orig. 6, 17, 25-27.