What is "drop of a sail"

Depth \Depth\ (s[e^]pth), n. [From Deep; akin to D. diepte, Icel. d[=y]pt, d[=y]p[eth], Goth. diupi[thorn]a.]

1. The quality of being deep; deepness; perpendicular measurement downward from the surface, or horizontal measurement backward from the front; as, the depth of a river; the depth of a body of troops.

2. Profoundness; extent or degree of intensity; abundance; completeness; as, depth of knowledge, or color.

   Mindful of that heavenly love Which knows no end in depth or height.
   --Keble.

3. Lowness; as, depth of sound.

4. That which is deep; a deep, or the deepest, part or place; the deep; the middle part; as, the depth of night, or of winter.

   From you unclouded depth above.
   --Keble.

   The depth closed me round about.
   --Jonah ii.

5. 5. (Logic) The number of simple elements which an abstract conception or notion includes; the comprehension or content.

6. (Horology) A pair of toothed wheels which work together.

7. (A["e]ronautics) The perpendicular distance from the chord to the farthest point of an arched surface.

8. (Computers) the maximum number of times a type of procedure is reiteratively called before the last call is exited; -- of subroutines or procedures which are reentrant; -- used of call stacks.

   Depth of a sail (Naut.), the extent of a square sail from the head rope to the foot rope; the length of the after leach of a staysail or boom sail; -- commonly called the drop of a sail.