APA Dictionary of Psychology

a sequence of numbers that do not indicate order, magnitude, or a true zero point but rather identify items as belonging to mutually exclusive categories. For example, a nominal scale for the performance of a specific group of people on a particular test might arbitrarily use the number 1 to denote pass and the number 2 to denote fail. Since the numbers represent category labels, they cannot be manipulated mathematically or otherwise quantitatively compared. A nominal scale is one of four types of measurement scale, the others being an ordinal scale, an interval scale, and a ratio scale. See also categorical scale.