Leading. (1) In photocomposition, the amount of vertical space between
lines of type, measured from baseline to baseline. In the United States
and the United Kingdom the measurement is given in Anglo-American points.
Most of Europe uses Didot points, with a few countries such as Germany
and Switzerland using the metric system. In phocomposition, leading is
synonymous with linefeed, baseline skip, or interline space; for example,
10/12 indicates 10-point type with a 12-point linefeed. The term has a
different usage in metal composition, where leading is the extra space
added to the body size. (2) With handseet metal type, the thin strip
of nonprinting brass, lead, or other material used to create additional
white space between lines of type.
Personally I prefer to use leading in its original meaning employed
with metal type, and to use a different term such as line separation or
line spacing to refer to the newer usage.