Meshlet
A small cluster of a model's triangles and vertices — typically up to 64 vertices and 126 triangles — that the GPU treats as a single unit.
Modern geometry pipelines split a mesh into hundreds of these clusters ahead of time. A mesh shader pulls one cluster at a time and can reject the whole packet with frustum, back-face and occlusion tests before a single vertex is shaded. NVIDIA introduced the concept with its Turing architecture in 2018; it now ships in DirectX 12 Ultimate and Vulkan.
The payoff is fine-grained visibility culling on the GPU itself rather than per whole object. Unreal Engine 5's Nanite builds on the same idea — geometry is partitioned into cluster hierarchies that pick a level of detail per cluster. The cost is a preprocessing step and the need for the entire tool chain to understand the new format.