OUGD404. Design Principles. Mini Task - Answer your Question.

what are picas, points, and pixels?

POINT.
a point is the smallest unit of typographic measurement. abbreviated to pt. the point is used to measure a font size i.e. the size of a letter form and the leading (spacing) between lines of text. the original size varied between 0.18 - 0.4 mm but now is commonly thought of as 0.35 - 0.38 mm according to wiki. other websites state 0.3302mm, 0.352mm (supposedly 1 postscript point), 

1 point (Truchet) = 0.188 mm (obsolete today)
1 point (Didot) = 0.376 mm = 1/72 of a French royal inch (27.07 mm)
1 point (ATA) = 0.3514598 mm = 0.013837 inch
1 point (TeX) = 0.3514598035 mm = 1/72.27 inch
1 point (Postscript) = 0.3527777778 mm = 1/72 inch
1 point (l’Imprimerie nationale, IN) = 0.4 mm
1 pica (ATA) = 4.2175176 mm = 12 points (ATA)
1 pica (TeX) = 4.217517642 mm = 12 points (TeX)
1 pica (Postscript) = 4.233333333 mm = 12 points (Postscript)
1 cicero = 4.531 mm = 12 points (Didot)

according to printwiki the american-english point is 0.351mm while the european point is 0.375mm. apparently the point reffers to the size of the metal block on which each letter form is cast, as opposed to the point being the actual size of the letter form (a small space must be allowed for each letter and its potential ascenders and descenders). so really point size is the distance between the ascender and descender.

PICA.
a pica is a typogrpahic unit of measurement that exists in conjunction with the point. it was originated around 1785 when François-Ambrose "L'éclat" Didot refined the typographic measures system created by Pierre Simon Fournier le Jeune. the american brittish pica:  0.166-inch pica and the 0.01383-inch point.  the european didot: the Didot system uses the cicero as its basic unit, which is equal to 12 corps, or 0.178 inch. The Didot corps measures 0.0148 inch. For general, practical measurement purposes, three decimal places (thousandths of an inch) are deemed sufficient significant digits. 


PIXEL.

Shorthand term for picture element, or the smallest point or dot on a computer monitor. Any computer display is divided into rows and columns of tiny dots, which are individual points at which the scanning electron beam has hit the phosphor-coated screen. The pixel is the smallest indivisible point of display on a monitor. The dot pitch is the measure of the diameter of an individual pixel; a monitor with a dot pitch of .28, for example, is composed of pixels .28 millimeter in diameter. The number of pixels per inch or lines of pixels per inch is a measure of screenresolution. It is commonly expressed as the horizontal and vertical dimensions of the pixel array; for example, a monitor described as 640 x 480 possesses 640 pixels across by 480 pixels down. The greater the number of pixels per inch, the better the resolution. The measure of the number of bits used to describe a pixel is known as color depth. (See Color Depth.) A pixel is also known as a pel, which is also short for picture element.