What is Inch?

Legal Definition
An inch (abbreviation: in or ) is a unit of length in the imperial and United States customary systems of measurement now formally equal to 136 yard but usually understood as 112 of a foot. Derived from the Roman uncia ("twelfth"), "inch" is also sometimes used to translate related units in other measurement systems, usually understood as deriving from the width of the human thumb. Traditional standards for the exact length of an inch have varied in the past, but since the adoption of the international yard during the 1950s and 1960s it has been based on the metric system and reckoned as exactly 2.54 cm.

-- Wikipedia
Legal Definition
A measure of length, containing one-twelfth part of a foot; originally supposed equal to three barleycorns.
See also
-- Black's Law Dictionary
Legal Definition
From the Latin uncia. A measure of length, containing one-twelfth part of a foot.
-- Bouviers Law Dictionary