Negative Space
The empty or unoccupied area surrounding the subject, which gives it room and defines its shape.
In depth
Negative space is not wasted space: it isolates a subject, gives the eye somewhere to rest, and often carries the emotional content of a picture — loneliness, calm, scale. It has to be genuinely empty to work, since any detail within it starts competing, which is why it usually means an unbroken sky, wall, water or shadow.
Examples in practice
A single figure occupying a tenth of the frame against a vast empty beach reads as solitary rather than merely small.