Symbolic simplex geometric region; stays unevaluated and is consumed by region functions (RegionDimension/RegionMeasure/…)
Simplex[{p1, …, pk}]
Graphics[Simplex[{{0, 0}, {1, 1}, {2, 0}}]]
→ -Graphics-Graphics3D[Simplex[{{0, 0, 1}, {1, 0, 0}, {1, 0, 1}, {1, 1, 1}}]]
→ -Graphics3D-Graphics3D[Simplex[3]]
→ -Graphics3D-