(TWentIeth of a Point) 1/20 of a Postscript point, or 1/1440th of an inch. There are thus 1440 twips to an inch or about 567 twips to a centimeter.
Twips are used in Microsoft formats and products, notably Rich Text Format, Visual BASIC, Visual C++, and printer drivers; and in IBM AFP products.
Twips were devised in the olden days to describe the sizes of characters produced by dot matrix printers that were constrained to multiples of either 12 or 10 dots per inch.
[Is it definitely relative to a __Postscript__ point, as opposed to one of the other definitions of point?]