In a triangle, if one of the angles is 90 degrees, this is a/an __________ triangle.

A triangle with one angle measuring 90 degrees is classified as a right triangle. This type of triangle is defined specifically by the presence of a right angle, which is characterized by the properties that arise from the Pythagorean theorem, stating that the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides. This differentiates right triangles from equilateral (where all angles are equal), isosceles (where at least two sides are of equal length), and scalene triangles (where all sides and angles are different). Therefore, identifying a triangle with a 90-degree angle as a right triangle is based on this fundamental geometric definition.