A dimensionless combination of several physical variables (e.g., velocity, density, viscosity), usually with a physical interpretation. Dimensionless groups arise naturally in the scale analysis of equations. The sixth edition of the McGraw–Hill Encyclopedia of Science and Technology lists 12 pages of dimensionless groups. See Boussinesq number, Cauchy number, Grashoff number, Mach number, Nusselt number, Péclet number, Prandtl number, Rayleigh number, Richardson number, Rossby number, Strouhal number, Taylor number; see also dimensional analysis, similarity theory, Buckingham Pi theory.