What is the most horrible name you could give to an arrangement of triangles? Well, another word for arrangement is a complex. And a triangle is the canonical simplex. So: a simplex complex? Its a cute name, but it apparently misses the point. The technical term is simplicial complex. Yeah, Simplicial. Who but a mathematician would determine "simple," and then "simplex," to be not-complex-enough?
Oh, a simplex is a shape that has exactly one more vertex than it has dimensions (like a triangle, and a pyramid). Its still synonymous with simple as an adjective.