Janus

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Roman Mythology The god of gates and doorways, depicted with two faces looking in opposite directions.
2. n. The satellite of Saturn that is sometimes the fifth and sometimes the sixth in distance from the planet. Periodically, Janus and another of Saturn's moons, Epimetheus, exchange orbits.

Century Dictionary and Cyclopedia

1. n. A primitive Italic solar divinity regarded among the Romans as the doorkeeper of heaven and the especial patron of the beginning and ending of all undertakings. As the protector of doors and gateways, he was represented as holding a staff or scepter in the right hand and a key in the left; and, as the god of the sun's rising and setting, he had two faces, one looking to the east, the other to the west. His temple at Rome was kept open in time of war and was closed only in the rare event of universal peace.
2. n. Hence A doorkeeper.
3. n. [NL.] A genus of hymenopterous insects of the family Uroceridæ, resembling Cephus, but distinguished from it by the filiform antennæ. There is one European species, J. connectus, and one North American, J. flaviventris.
4. n. A double monster with the two heads looking in opposite directions. Also called janiceps.

Wiktionary

1. n. Roman mythology The god of gates and doorways; having two faces looking in opposite directions.
2. n. astronomy A moon of Saturn.

GNU Webster's 1913

1. n. (Rom. Antiq.) A Latin deity represented with two faces looking in opposite directions. Numa is said to have dedicated to Janus the covered passage at Rome, near the Forum, which is usually called the Temple of Janus. This passage was open in war and closed in peace.

WordNet 3.0

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