The inch is a unit of length in the British Imperial and the United States customary systems of measurement. It is equal to 1/36 yard or 1/12 of a foot. Derived from the Roman uncia ("twelfth"), the word inch is also sometimes used to translate similar units in other measurement systems, usually understood as deriving from the width of the human thumb.
A measuring tape with inches
A fire hydrant marked as 3-inch
Mid-19th-century tool for converting between different standards of the inch
A unit of measurement, or unit of measure, is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multiple of the unit of measurement.
The former Weights and Measures office in Seven Sisters, London
Units of measurement, Palazzo della Ragione, Padua
An example of metrication in 1860 when Tuscany became part of modern Italy (ex. one "libbra" = 339.54 grams)