RecurSim is a portmanteau of Recursive and Simplex.
From Wikipedia:
A simplex is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions.
A recursive simplex or a fractal simplex is a structure that’s constructed by copies of smaller versions of itself. Examples include the Sierpinski triangle:
Sierpinski triangle
My favorite recursim is the Sierpinski tetrahedron or Tetrix which has a very interesting fact about its dimension.
Sierpinski tetrahedron
Here’s a Numberphile video if you want to dig more on this topic:
Spoiler
The Sierpinski tetrahedron is a fractal object that appears 3D but actually has a nice integer dimension of 2!
For the nitpickers out there, yes, I know that a “recursive simplex” is not strictly a simplex but it’s rather a hollowed out simplex and a fractal object but I take liberty in the terminology.