In archaeology, a biface is a two-sided stone tool, manufactured through a process of lithic reduction, that displays flake scars on both sides. Bifacial artifacts can be made on large flakes or lithic cores, and may be grouped into numerous distinct classes. For the purposes of this article, four classes are defined:

• Class I consists of large, thick bifaces reduced from cores or thick flakes; these are referred to as blanks.
• Class II consists of thinned blanks. While form remains rough and uncertain, an effort has been made to reduce the thickness of the flake or core.
• Class III bifaces may be either preforms or crude formalized tools, such as adzes.
• Class IV includes the finer formalized tool types such as projectile points and fine bifaces.

It must be emphasized that, while Class IV bifaces are referred to as "formalized tools", bifaces from any stage of a lithic reduction sequence may be used as tools.