Sentence:  We have seen that under any reasonable definition of area, the <WSD>angular defect</WSD> of a triangle is proportional to its area. 
Meaning of the word marked with <WSD> tags:  (non-Euclidean geometry) The amount by which the total of the interior angles of a triangle is less than 180° (π radians); the amount by which the total of the internal angles of a polygon is less than what would be expected on the Euclidean plane.