The crossword solution for the term 90-DEGREE ANGLE is ELL, which is an abbreviation for the word 'ELLIPSE'. An ELLIPSE is a closed curve in a plane where the sum of the distances from any point on the curve to two fixed points is constant. A 90-degree angle is a right angle, which is an angle that measures exactly 90 degrees. It is one of the most common angles used in geometry and is formed when two straight lines intersect at a single point.