That's a "rhombus".
(We know it's not a rectangle, because a rectangle
doesn't necessarily have all equal sides.)
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It is a rhombus
All rectangles have 4 sides. All parallelograms have 4 sides. A rectangle is a parallelogram with all 4 angles of 90°
Rectangle, parallelogram
A rectangle may be called a parallelogram but they are both 4 sided quadrilaterals. A parallelogram can be called a rectangle if all four angles are right angles (90 degrees). A parallelogram can be called a rhombus if all four sides are the same size. A parallelogram can be called a square if all four angles are right angles and all four sides are the same size
That would be a rhombus. (It can't be a 'square', because a square is also a rectangle.)