What is a triangle that includes one obtuse angle called?

A triangle that includes one obtuse angle is known as an obtuse triangle. By definition, an obtuse angle is one that measures more than 90 degrees but less than 180 degrees. In an obtuse triangle, one of the internal angles exceeds 90 degrees, which classifies the triangle as obtuse. The other two angles in the triangle must be acute, meaning they each measure less than 90 degrees, because the sum of all angles in any triangle always equals 180 degrees.

In contrast, an acute triangle contains only angles that are less than 90 degrees, while an equilateral triangle has all three angles equal to 60 degrees, and a right triangle features one angle exactly equal to 90 degrees. Thus, since the defining characteristic of an obtuse triangle is the presence of an angle greater than 90 degrees, it is correctly identified as an obtuse triangle.