depth

Concise Oxford English Dictionary © 2008 Oxford University Press:
depth/depθ/

▶noun

• 1 the distance from the top or surface to the bottom of something or to a specified point within it.

  ■ the distance from the front to the back of something.

  ■ the apparent existence of three dimensions in a two-dimensional representation.

• 2 the quality of being intense or extreme.

  ■ complexity and profundity of thought.

  ■ comprehensiveness of study or detail: third-year courses go into more depth.

• 3 (the depths) the deepest or lowest part of something.

– phrases
hidden depths previously unnoticed qualities.
in depth in great detail; comprehensively and thoroughly.
out of one's depth

• 1 in water too deep to stand in.

• 2 in a situation beyond one's capabilities.

– origin ME: from deep + -th2, on the pattern of pairs such as long, length.