Positive Space
The area of the frame occupied by the subject itself.
In depth
The ratio between positive and negative space is one of the fundamental compositional decisions: dominant positive space produces immediacy and pressure, while a small positive area against a large empty one produces context and distance. Neither is correct, but a roughly equal split is often the weakest choice because it commits to neither reading.
Examples in practice
Filling nine-tenths of the frame with a face produces intimacy; the same face at a tenth produces isolation.