Triangle strip
A primitive where each vertex after the first two forms a triangle with the previous two, so N triangles cost N+2 vertices instead of 3N.
Every vertex after the opening two closes a triangle against its two predecessors, with the winding order alternating automatically on each triangle. A strip of N triangles is N+2 vertices — a large saving over an unindexed list's 3N when geometry is fed through a narrow vertex pipeline. Separate strips can be stitched into one draw call using degenerate (zero-area) triangles that render nothing.
The close relative is the triangle fan, which shares one hub vertex. Modern engines usually prefer indexed triangle lists, where the post-transform vertex cache recovers almost the same reuse without the strip's authoring constraints; strips survive mainly in bandwidth-bound or legacy paths and in vertex buffers on simple hardware.