A unit of length defined (in the US at least) to be 35/83 cm (or approximately 1/6th of an inch). This was used for measuring the length of lines of text (as "30 picas and 4 points long"), but not for measuring font heights. 6 picas equal 1 inch.
A point is a unit of measurement most commonly used in the measurement of type. 12 points are equal to one picas. 72 points are equal to 1 inch.
In traditional typography a 10pt font was one where the block of metal for each glyph was 10 points high. The point size of a font is the unleaded baseline to baseline distance.
In composition, a printer's tool calibrated in picas and points used for type measurement.