AbsoluteThickness[d] is a graphics directive which specifies that lines which follow are to be drawn with absolute thickness d.
The absolute thickness is measured in units of printer's points, approximately equal to of an inch.
AbsoluteThickness can be used in both two- and three-dimensional graphics.
See The Mathematica Book: Section 2.9.3.
See also: Offset, AbsoluteDashing, AbsolutePointSize, PointSize, Dashing.