According to Ching in “A Visual Dictionary of Architecture,”
Any in a series of standardized, frequently interchangeable components used in assembling units of differing size, complexity, or function. A unit of measurement used for standardizing the dimensions of building materials or regulating the proportions of an architectural composition.
- Ching, Francis D.K. A Visual Dictionary of Architecture (New York: John Wiley and Sons, 1995), 50 and 56.