Real Dictionary

depth
In photography and art, you say that a picture has depth or depth of field when you mean that it appears three-dimensional rather than flat.
The depth of something such as a cupboard or drawer is the distance between its front surface and its back.
The depth of something such as a river or hole is the distance downwards from its top surface, or between its upper and lower surfaces.
uncountable noun: The depth of a colour is its intensity and strength.
uncountable noun: The depth of a situation is its extent and seriousness.
uncountable noun: The depth of someone's knowledge is the great amount that they know.
variable noun: If an emotion is very strongly or intensely felt, you can talk about its depth.

depth in American English
the distance from the top downward, from the surface inward, or from front to back
noun: a dimension taken through an object or body of material, usually downward from an upper surface, horizontally inward from an outer surface, or from top to bottom of something regarded as one of several layers

depth in British English
noun: the extent, measurement, or distance downwards, backwards, or inwards