Janus, n. (1508)
1. The name of an ancient Italian deity, regarded as the doorkeeper of heaven, as guardian of doors and gates, and as presiding over the entrance upon or beginning of things; represented with a face on the front and another on the back of his head; the doors of his temple in the Roman Forum were always open in time of war, and shut in time of peace. Often used allusively, and in attributive and other relations.
2. a. Designating materials with a double facing, or things having a two-way action.