A dimensionless group is any combination of dimensional or dimensionless quantities possessing zero overall dimensions. Dimensionless groups are frequently encountered in engineering studies of complicated processes or as similarity criteria in model studies. A typical dimensionless group is the Reynolds number (a dynamic similarity criterion), Re = vlρ/μ. Since the dimensions of the quantities involved are velocity v: (L/θ); characteristic length l: (L); density ρ: (M/L³); and viscosity μ: (M/LT) (with M, L, and T as the fundamental units of mass, length, and time), the Reynolds number reduces to a dimensionless group and can be represented by a pure number in any coherent system of units.