Lewis & Short

Jānus, i, m. [root i, ire, prop. a going, a going through, passage; cf. janua],

  1. I. an old Italian deity. He was represented with a face on the front and another on the back of his head, Ov. F. 1, 245; hence, anceps, id. M. 14, 334. The month of January, Mensis Jani, Ov. F. 2, 51, was sacred to him, as were all other beginnings. The myth makes him a king of Latium or Etruria, where he hospitably received Saturn when expelled by Jupiter from Crete, Macr. S. 1, 7, 8, 9. He had a small temple in the Forum, with two doors opposite to each other, which in time of war stood open and in time of peace were shut; the temple was thrice closed on this account: in the time of Numa, after the first Punic war, and after the battle of Actium, Ov. F. 1, 281. With reference to his temple, the deity was called Janus geminus, or Janus Quirinus, Macr. S. 1, 9; Suet. Aug. 22; for which, poet.: Janus Quirini, Hor. C. 4, 15, 9.
    Joined with pater: Januspater, Gell. 5, 12, 5.
  2. II. Transf.
    1. A. The temple of Janus: Janum ad infimum Argiletum indicem pacis bellique fecit, Liv. 1, 19; cf. Hor. Ep. 1, 20, 1.
    2. B. An arched passageway, covered passage, arcade: janos tres faciendos locavit, Liv. 41, 27; 2, 49; Cic. N. D. 2, 27, 67; Suet. Aug. 31.
    3. C. In partic., Jani, four arched passages in the Roman Forum, where the merchants and moneychangers had their stand: qui Puteal Janumque timet, celeresque Calendas, Ov. R. Am. 561: haec Janus summus ab imo Prodocet, Hor. Ep. 1, 1, 54: postquam omnis res mea Janum Ad medium fracta est, id. S. 2, 3, 18; Cic. Off. 2, 25, 90; id. Phil. 6, 5, 15 al.
    4. D. A year: vive, vale et totidem venturos congere Janos, quot, etc., Aus. Ep. 20, 13.