The Ewald sphere is a geometric construction used in electron, neutron, and x-ray diffraction which shows the relationship between: 
* the wavevector of the incident and diffracted beams, 
* the diffraction angle for a given reflection, 
* the reciprocal lattice of the crystal.
It was conceived by Paul Peter Ewald, a German physicist and crystallographer. Ewald himself spoke of the sphere of reflection. It is often simplified to the two-dimensional "Ewald's circle" model or may be referred to as the Ewald sphere.