Is a square a rhombus?

Yes, a square is a rhombus. But a rhombus is not a square unless the interior angles are at right angles.

We know that, a square is a quadrilateral with all sides equal and all the interior angles are 90 deg i.e, at right angles. 

Whereas

a rhombus is also a quadrilateral with all sides equal but the interior angles are not at right angles. Hence, if the interior angles of a rhombus are all at right angles (90 deg), then it is square.