The name "isosceles" originated in the mid-18th century. It comes from the Greek word isoskeles, which is made up of two words: "isos," meaning "equal," and "skelos," meaning "leg." The complete word means "having two sides of equal length." This is the defining property of an isosceles triangle.
Any triangle that has two or more sides of equal length is an isosceles triangle. The third side can be shorter or longer than the two that are the same. It can even be equal to them; an equilateral triangle is one that has three equal sides and is a special case of an isosceles triangle. When an isosceles triangle is drawn, it is usually the base of the triangle that is different in length.
An isosceles triangle has two equal angles as well as two equal sides. If the base of the triangle is the longer or shorter side, then the two base angles are equal. Another way to find which of the three angles inside the triangle are equal is to find the equal sides, which you can usually do by eye. It is the angles opposite these sides that are equal. All three angles in the special case of an equilateral triangle are the same.
Unless they are also equilateral, isosceles triangles have only one plane of symmetry. If the triangle is arranged so that the side of different length is the base, a line drawn from the top point of the triangle to the midpoint of the base is the triangle's line of symmetry. One unique property of isosceles triangles is that the line of symmetry is also the bisector of the angle between the two equal sides and perpendicular bisector of the base. A bisector is a line that cuts something in half, and a perpendicular bisector is a line at right angles to the thing it is cutting in half.