A triangle that has three equal sides is called what?

A triangle that has three equal sides is called an equilateral triangle. This is because, by definition, an equilateral triangle has all three sides of the same length, which also leads to all three internal angles being equal, each measuring 60 degrees. This specific property of having equal sides and angles distinguishes it from other types of triangles.

In contrast, an acute triangle is characterized by all angles being less than 90 degrees, but it does not specify anything about the sides being equal. An isosceles triangle has at least two sides that are equal in length, but not necessarily all three. A scalene triangle, on the other hand, has all sides of different lengths, and therefore cannot be equilateral. Thus, the definition and characteristics of an equilateral triangle clearly establish why this is the correct answer.