The glancing angle is defined as the angle between the incident ray( or beam) and the surface. An example of a glancing angle is a beam of electrons incident on the crystal.
Also, there is a glancing angle of reflection and the glancing angle of incidence.
The glancing angle of reflection is the angle between the surface and the reflected ray (or beam). It is the component of the angle of reflection.
The glancing angle of incidence is the angle between an incident beam and the surface. It is the component of the angle of incidence.