A right-angled triangle is a type of triangle that has one angle measuring exactly 90 degrees. This right angle is typically marked with a small square in diagrams. The side opposite the right angle is called the hypotenuse, while the other two sides are referred to as the legs. Right-angled triangles are important in geometry and trigonometry, as they follow the Pythagorean theorem, which states that the square of the hypotenuse's length is equal to the sum of the squares of the lengths of the other two sides. This property makes them useful in various applications, including construction and navigation.