1. adj. (heading, of a physical distance) Extending far away from a point of reference, especially downwards.
2. adj. Extending far down from the top or surface; having its bottom far down.
We hiked into a deep valley between tall mountains.
There was a deep layer of dust on the floor; the room had not been disturbed for many years.
3. adj. Far in extent in another (non-downwards, but generally also non-upwards) direction away from a point of reference.
The shelves are 30 centimetres deep. — They are deep shelves.
4. adj. In a (specified) number of rows or layers.
a crowd three deep along the funeral procession
5. adj. Thick.
That cyclist's deep chest allows him to draw more air.
6. adj. Voluminous.
to take a deep breath / sigh / drink
7. adj. A long way inside; situated far in or back.
deep into the forest; deep in the forest
8. adj. # (cricket, baseball, softball) Far from the center of the playing area, near to the boundary of the playing area, either in absolute terms or relative