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The triangle characterized by one angle that is exactly 90 degrees is known as a right triangle. This type of triangle is defined by the presence of this right angle, which is always 90 degrees, while the other two angles must be acute, meaning they each measure less than 90 degrees.

In geometry, right triangles are important because they are used in various applications, including illustrating the Pythagorean theorem, which relates the lengths of the sides of right triangles. The side opposite the right angle is known as the hypotenuse, and it is the longest side of the triangle.

In contrast, an obtuse triangle contains one angle that is greater than 90 degrees, while an acute triangle has all angles measuring less than 90 degrees. An equilateral triangle consists of three equal sides and three angles that each measure 60 degrees, certainly not including a right angle at all. Therefore, the defining feature that sets a right triangle apart is its one right angle, affirming the correctness of identifying it as the answer.