1. A measure of length, the twelfth part of a foot, commonly subdivided into halves, quarters, eights, sixteenths, etc, as among mechanics. It was also formerly divided into twelve parts, called lines, and originally into three parts, called barleycorns, its length supposed to have been determined from three grains of barley placed end to end lengthwise. It is also sometimes called a prime ('), composed of twelve seconds (), as in the duodecimal system of arithmetic. 12 seconds () make 1 inch or prime. 12 inches or primes (') make 1 foot. (B. Greenleaf)
2. A small distance or degree, whether or time space; hence, a critical moment. Beldame, i think we watched you at an inch. (Shak) By inches, by slow degrees, gradually. Inch of candle. See candle. Inches of pressure, usually, the pressure indicated by so many inches of a mercury column, as on a steam gauge. Inch of water. See water.