Conditions imposed on an (unknown) solution of a differential equation on the boundary (or on a part of the boundary) of the domain in which the solution is to be determined. Boundary conditions are usually formulated in terms of differential operators, though other types are also encountered.
Boundary conditions. A.P. Soldatov (originator), Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Boundary_conditions&oldid=12691