An equilateral triangle is a triangle with all three sides of equal length, corresponding to what could also be known as a "regular" triangle. An equilateral triangle is therefore a special case of an isosceles triangle having not just two, but all three sides equal. An equilateral triangle also has three equal 60 degrees angles. The altitude h of an equilateral triangle is h=a sin 60°=1/2 √3 a.

Source: Math World