Definition
Rhombus
A rhombus is a quadrilateral with four sides that are all the same length.
What it means
Check the four sides, not the way the shape is tilted. If every side has the same length, the quadrilateral is a rhombus. Its corners do not have to be right angles. A square counts too, because it follows the same four-equal-sides rule.
Remember it
Four equal sides are the password; turning the shape cannot change that.
Example
A quadrilateral has side lengths 6 cm, 6 cm, 6 cm, and 6 cm. It is a rhombus. If all four corners are also right angles, the same shape is also a square.
Watch for
A shape that merely looks like a diamond is not enough; without four equal sides, it is not a rhombus.
Common mix-up
A square is not outside the rhombus family. It is the rhombus with four right angles.
Exact meaning
A rhombus is a quadrilateral whose four side lengths are equal.