The Calvin and Hobbes Wiki

The only visual of the 5th dimension

The 5th DImension is a separate dimension in the Spaceman Spiff universe. The only thing we know about it is that time has no meaning, and that it is beyond human comprehension. The only known way to access the 5th Dimension is through the use of Hyper-jets, which one model was installed on Spaceman Spiff's spaceship.

Real Life Meaning

The 5th Dimension was analogous to Calvin feeling that class was taking forever.