CSG (constructive solid geometry)
Build complex solids by adding and subtracting simple ones — union, intersection and difference of boxes and spheres.
Constructive Solid Geometry was formalised in CAD research in the late 1970s. A CSG model is a tree: the leaves are primitives (cube, sphere, cylinder) and the nodes are Boolean operations. Ray tracers can evaluate the tree directly, without first turning it into surfaces.
In games CSG shows up in level editors: both the Quake and Unreal editors build geometry by adding and subtracting "brushes", which then compile down to BSP trees. Blender's boolean modifier is the same idea. Cheap to sketch with, but the result usually needs cleanup — overlapping faces and broken normals come along for free.