Wiktionary
n. the quality, of depth or thickness, that differentiates a solid object from a two-dimensional image of it
WordNet
n. the dimension whereby a solid object differs from a two-dimensional drawing of it
Usage examples of "third dimension".
Square, who lives in the Euclidean plane, a twodimensional society of triangles, hexagons and circles, and doesn't believe in the third dimension until a passing sphere drops him in it.
When this third dimension is pointed out to us it explains so many phenomena in such a simple fashion that our first instincts are to reject it, to return to our more familiar restrictive world of anomaly and paradox.
First there is the difficulty that for us, light oscillates in a plane, and moves forward in the third dimension.
To turn one into its mirror image I had to lift it up and rotate it in the third dimension.
However, if you were able to travel in the third dimension, you could cut straight across.
If the sphere rotates slowly, they move slowly and steadily into the third dimension, which appears to their consciousness as time.
That shortcut goes through the third dimension, something that a Flatlander living on the strip might have a tough time visualizing.
The Stars no longer spread as upon a Dom'd Surface, he now beholds them in the Third Dimension as well, the Eye creating its own Zed-Axis, along which the star-chok'd depths near and far rush both inward and away, and soon, quite soon, billowing out of control.
Economics lay along one axis, social indices along another, with politics making up the third dimension.