What type of triangle has one angle measuring 90 degrees?

A triangle with one angle measuring 90 degrees is classified as a right triangle. This definition is fundamental in geometry, where a right triangle is characterized specifically by the presence of a right angle, which is an angle that measures exactly 90 degrees.

Right triangles are unique in that they have properties related to the Pythagorean theorem, which states that in a right triangle, the sum of the squares of the lengths of the two shorter sides (the legs) is equal to the square of the length of the longest side (the hypotenuse). This relationship is crucial in various applications including calculating distances, determining heights, and solving real-world problems involving right angles.

In contrast, other types of triangles do not meet the criteria for having a right angle. An isosceles triangle has two sides of equal length but does not necessarily include a right angle. A scalene triangle has all sides of different lengths and does not guarantee the presence of a right angle either. An equilateral triangle has all sides and all angles equal, with each angle measuring 60 degrees, which also does not include a right angle. Thus, the defining feature of a right triangle is its 90-degree angle, making it the correct choice in this context.