Inch: In length, 1/12th of foot or 1/36 of a yard or, metrically, 2.54 centimeters. The inch, along with the foot and yard, are Old World creations to which the USA has stubbornly clung. The inch was originally about the length of the last bone (distal phalanx) in a man's thumb and served as a measurement of land. The etymology (word history) of "inch" is remarkable. It originally meant "one twelfth". The abbreviation is "in."