Because it has 4 equal sides.
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No. That is not enough to make it a regular quadrilateral. A regular polygon must also have equal angles and a rhombus, in general, does not.
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A rhombus
Square and a rhombus * * * * * A rhombus is not a regular quadrilateral (unless it is a square) because being regular requires all sides AND all angles to be equal. So there is only one type of a regular quadrilateral - a square. It may have other names, certainly so in other languages.
A rhombus is a quadrilateral.
A square is a rectangle, a rhombus, an equilateral parallelogram, and a regular quadrilateral.
A rhombus is a quadrilateral because it has four sides. A quadrilateral with four equal sides is a rhombus.