A shape is equilateral if all the sides are the same length.
In geometry class, people learn about many shapes, such as triangles and squares. A square is equilateral, because all of its sides are the same length. A rhombus is also equilateral — its sides are also the same length. A triangle can be equilateral too, but not a scalene triangle, which has three sides of different lengths. The word equal should help you remember what equilateral means. You can also use it as a noun for an equilateral shape.