ancient Italic deity, to the Romans the guardian god of portals, doors, and gates; patron of beginnings and endings, c. 1500, from Latin Ianus, literally "gate, arched passageway," perhaps from PIE root *ei- "to go" (cognates: Sanskrit yanah "path," Old Church Slavonic jado "to travel"). He is shown as having two faces, one in front the other in back (they may represent sunrise and sunset and reflect an original role as a solar deity). His temple in Rome was closed only in times of peace. Related: Janian.

Others are reading

Definitions of Janus from WordNet

Janus (n.)
(Roman mythology) the Roman god of doorways and passages; is depicted with two faces on opposite sides of his head;