Elasticity of substitution

Elasticity of substitution is the elasticity of the ratio of two inputs to a production (or utility) function with respect to the ratio of their marginal products (or utilities). It measures the curvature of an isoquant and thus, the substitutability between inputs (or goods), i.e. how easy it is to substitute one input (or good) for the other. In the modern period, John Hicks is considered to have formally introduced this concept in 1932, however he had, by his own admission, introduced the inverse of the elasticity of substitution, or the elasticity of complementarity. The credit then, also by Hicks' own admission, should go to Joan Robinson.