The pica is a unit of length in the field of typography and uses the symbol pc.
There are 12 points in 1 pica. 1 pica represents approximately 1/6 of an inch.
Picas are expressed using numbers and a lower case p on software such as Adobe InDesign - for example 8p6 which would represent 8 picas and 6 points (or 8.5 picas).
The Pica is very useful in measuring and calculating the space needed to print letters and words on paper - for example when laying out books or magazine articles; the font lengths are quanitified by the number of characters per pica (cpp).
The metre (or meter; US spelling) is the SI base unit of length and uses the symbol m.
Originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole but was redefined in 1799 in terms of a prototyped metre bar (this bar was changed again in 1889).
In 1983 the current definition was adopted and it is now the most common unit in any property in the world and is used in thousands of formulae to describe countless characteristics.
Let's hear it for the metre!