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late 13c., "forehead," from Old French front "forehead, brow" (12c.), from Latin frontem (nominative frons) "forehead, brow, front; facade, forepart; appearance," perhaps literally "that which projects," from PIE *bhront-, from root *bhren- "to project, stand out." Or from PIE *ser-, "base of prepositions and preverbs with the basic meaning 'above, over, up, upper'" [Watkins].
Sense of "foremost part of anything" developed in Latin. The military sense of "foremost part of an army" (mid-14c.) led to the meaning "field of operations in contact with the enemy" (1660s). Home front is from 1919. Sense of "public facade" is from 1891; that of "something serving as a cover for illegal activities" is from 1905. Meteorological sense first recorded 1921. Front yard first attested 1767.
1520s, from Middle French fronter, from Old French front (see front (n.)). Related: Fronted; fronting.
The boundary between two air masses that have different temperatures or humidity. In the mid-latitude areas of the Earth, where warm tropical air meets cooler polar air, the systems of fronts define the weather and often cause precipitation to form. Warm air, being lighter than cold air, tends to rise, cool, and condense along such boundaries, forming rain or snow. See also cold front, occluded front, polar front, stationary front, warm front.