Definition of "tri"

The most common poly budget in use for games at the time of this writing is between 5,000 and 10,000 tris. Anything within that range is sufficient to accommodate all superficial anatomical details without resort to optimization beyond normal model cleanup.

2009, Andrew Paquette, Computer Graphics for Artists II: Environments and Characters

Tris and quads have different areas of functionality. In real-time graphics, tris are the norm because they provide the most basic geometric representations of planes.

2010, Tony Mullen, Claudio Andaur, Blender Studio Projects: Digital Movie-Making, page 91