In digital typography, the body height refers to the distance between the top of the tallest letterform to the bottom of the lowest one.
In manual typesetting, the body height or the font (or point) size is defined by the height of the lead cuboid (metal sort) on which the actual font face is moulded. The body height of a metal sort is a few points bigger than the actual distance between the ascender and descender lines to allow additional space between the lines of text. More space might be achieved by inserting thin long pieces of lead between the lines of text (that is leading).
Following the tradition, the point size in digital fonts may be defined as the distance between the ascender and descender lines plus a few points above and below the ascender and descender lines, respectively.