Definitions for "Stencil buffer"
Keywords:  buffer, depth, mask, pixel, bitplanes
A Stencil Buffer is an improvement on the Z-Buffer, and allows depth information to be stored at values of 1, 2, 4, and 6 levels.
The stencil buffer is not the same buffer as the Z buffer and depth buffer. The stencil buffer is used to restrict drawing to certain portions of the screen.
an additional buffer of per-pixel information, much like a z-buffer
It allows to map a secondary image on the principal one contained in a framebuffer.