Sentence:  The <WSD>angle</WSD> between lines A and B is π/4 radians, or 45 degrees. 
Meaning of the word marked with <WSD> tags:  (measure of such a figure) The measure of such a figure. In the case of a plane angle, this is the ratio (or proportional to the ratio) of the arc length to the radius of a section of a circle cut by the two rays, centered at their common point. In the case of a solid angle, this is the ratio of the surface area to the square of the radius of the section of a sphere.