Counterform (Negative Space)
A meaningful shape produced primarily by the empty area around or between positive forms, carrying real visual weight of its own.
In practice
Negative space is the empty area around and between shapes; a counterform is that emptiness when it forms a shape worth reading. The inside of the letter O and the gap between two letters are counterforms. Designers use them deliberately: a logo where the space between two letters forms an arrow, or a poster where two facing profiles appear in the white area around a vase.
Often confused with Space
Space is the whole area, filled or empty, in and around forms; a counterform is a specific empty shape that reads as meaningful in its own right.
Try this
Cut a shape out of black paper and look only at the hole it leaves; describe what that hole resembles.