Tesseract

Line = L = one dimension = length

Square = L squared = two dimensions = area

Cube = L cubed = three dimensions = volume

Tesseract = L to the fourth degree = four dimensions = hypercube (cube within a cube)

The professor calls it the "four-dimensional version of a cube."

Having fun? You should read about tesseracts in A Wrinkle in Time by Madeleine L'Engle.