1. The depth or thickness of a real object.
2. A metaphor for LSD, at least in the minds of certain cartoonists.
"The third dimension is a theoretical realm of space and time in which the particles and dark matter of this parallel, alternate reality bends light to collide with the electrical charges of the subconscious mind. What is light becomes dark. What is dark becomes light. Some look into the third dimension and see nothingness; others believe they see the very face of God." - Fluffy Thing, Intermission in the Third Dimension