Janus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The god of gates and doorways, depicted with two faces looking in opposite directions.

from The Century Dictionary.

• noun A double monster with the two heads looking in opposite directions. Also called janiceps.
• noun 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.
• noun Hence A doorkeeper.
• noun [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.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.
• noun a fabric having both sides dressed, the sides being of different colors, -- used for reversible garments.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun Roman mythology The god of gates and doorways; having two faces looking in opposite directions.
• proper noun astronomy A moon of Saturn.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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