Boundary Conditions of a Partial Differential Equation which are a weighted Average of Dirichlet Boundary Conditions (which specify the value of the function on a surface) and Neumann Boundary Conditions (which specify the normal derivative of the function on a surface).
See also Boundary Conditions, Cauchy Problem, Dirichlet Boundary Conditions, Neumann Boundary Conditions
References
Morse, P. M. and Feshbach, H. Methods of Theoretical Physics, Part I. New York: McGraw-Hill, pp. 678-679, 1953.