A pica is a typesetting unit of measurement commonly used for measuring lines of type. One pica is 12 points and is approximately 1/6 of an inch.
More precisely a pica is .166 inch or 4.2154 millimeters and six picas equals .9957 inches but for most graphic design and desktop publishing purposes six picas are rounded up to one inch.
The letter p is used to designate picas as in 22p or 6p. With 12 points to the pica, half a pica would be 6 points written as 0p6. 17 points would be 1p5 (1 pica = 12 pts, plus the leftover 5 pts).