A pica (pronounced /ˈpaɪkə/) is a typographic unit of measure corresponding to 1/72nd of its respective foot, and therefore to 1/6th of an inch. The pica contains 12 point units of measure.
The pica originated around 1785, when Françoise "L'éclat" Ambrose Didot (1730–1804) refined the typographic measures system created by Pierre Simon Fournier le Jeune (1712–1768). He replaced the traditional measures of cicéro, PetitRoman, and GrosText with “tenpoint”, “twelvepoint”, et cetera.
To date, in printing these three pica measures are used:
Usually, pica measurements are represented with an uppercase "P" with an upperrighttolowerleft virgule (slash) starting in the upper right portion of the "P" and ending at the lower left of the upright portion of the "P"; essentially drawing a virgule ( / ) through a "P". (P̸) Likewise, points are represented with number of points before a lowercase "p", for example, 5p represents “5 points”, and 6P2p represents “6 picas and 2 points”, and 1P1 represents “13 points”, which is converted to a mixed fraction of 1 pica and 1 point.
Publishing applications such as Adobe InDesign and QuarkXPress represent pica measurements with wholenumber picas left of a lowercase "p", followed by the pointsnumber, for example: 5p6, represents 5 picas and 6 points, or 5½ picas.
Cascading Style Sheets defined by w3c use "pc' for the abbreviation for pica. Also
pt: points  the points used by CSS2 are equal to 1/72^{nd} of an inch.
pc: picas  1 pica is equal to 12 points.
Note that these definitions are different from a typewriter's pica setting, which denotes a type size of ten characters per horizontal inch.

