Tri (Triangle)
A three-vertex face, the minimum possible polygon.
A tri is a face made of exactly three vertices, the minimum needed to define a surface. Everything eventually becomes triangles when rendered or exported to a game engine, which is why asset specs quote triangle counts. Modelers usually work in quads and let the software triangulate at the end.
These fundamentals are covered hands-on in Blender Geometry Essentials 01, free with a free account.