A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.
In its most common form, a cube, it is made from two triangular glass prisms which are glued together at their base using Canada balsam. The thickness of the resin layer is adjusted such that (for a certain wavelength) half of the light incident through one "port" (i.e. face of the cube) is reflected and the other half is transmitted due to Frustrated Total Internal Reflection. Polarizing beam splitters, such as the Wollaston prism, use birefringent materials, splitting light into beams of differing polarization.