one of the principal methods of dimensional analysis, introduced by the American physicist Edgar Buckingham in 1914. The theorem states that if a variable A₁ depends upon the independent variables A₂, A₃, . . . , A_n, then the functional relationship can be set equal to zero in the form f(A₁, A₂, A₃, . . . , A_n) = 0. If these n variables can be described in terms of m dimensional units, then the pi (π) theorem states that they can be grouped in n - m dimensionless terms that are called π-terms—that is, ϕ(π₁, π₂, π₃, . . . , π_{n - m}) = 0. Further, each π-term will contain m + 1 variables, only one of which need be changed from term to term.

The utility of the pi theorem is evident from an example in fluid mechanics. To investigate the characteristics of fluid motion and the influence of the variables involved, it is possible to group the important variables in three categories, namely: (1) four linear dimensions that define channel geometry and other boundary conditions, (2) a rate of water discharge and a pressure gradient that characterize kinematic and dynamic flow properties, and (3) five fluid properties—density, specific weight, viscosity, surface tension, and elastic modulus. This total of 11 variables (n) can be expressed in terms of three dimensions (m); accordingly, a functional relationship can be written involving eight π-terms (n - m). The problem is reducible to solution of simultaneous linear equations to determine the exponents of the π-terms that will render each term dimensionless—i.e., π_i = L⁰M⁰T⁰, in which L⁰, M⁰, and T⁰ refer to a dimensionless combination of length, mass, and time, the three fundamental units in which each variable is described.

The interesting result of this algebraic exercise is E = kϕ(a, b, c, F, R, W, C), in which E is the Euler number, characterizing the basic flow pattern, k is a constant, and ϕ expresses the functional relationship between E and a, b, c (parameters defining the boundary characteristics), and F, R, W, and C. The latter are the dimensionless Froude, Reynolds, Weber, and Cauchy numbers that relate fluid motion to the properties of weight, viscosity, surface tension, and elasticity, respectively.