A rhombus is an equilateral parallelogram, which is a two-dimensional, four-sided figure in which opposite sides are parallel and all sides have equal length. A square fits this definition. However, it is usually considered a special case, and some definitions of a rhombus specify that squares are not included.
Without a specific exclusion, a square qualifies not just as a rhombus but also as a specific type of rectangle, since it is a four-sided figure with four congruent, 90-degree angles. In fact, squares, rectangles and rhombuses can all be considered specific types of parallelograms. However, even if all squares are rhombuses, not all rhombuses are squares.