An equilateral triangle is a triangle with three sides of equal length and three equal angles. It is a regular polygon, occasionally known as the regular triangle. It is a special case of an isosceles triangle by modern definition. Its internal angle is 60° in degree or π/3 in radian. Its perimeter is three times the side length. Its area is a quarter of the square root of three multiplied with its squared side length. Its appearance is unchanged by three reflection lines, each of which passing through a vertex to the midpoint of the opposite edge, and three rotations around its centroid.