Is a square a rhombus?

Yes. By definition, a rhombus is simply a shape that has four equal sides. Therefore a square is a rhombus.

check that perchik. A rectangle has all equal (right) angles. A rhombus has all equal sides. A square has all equal sides AND all equal angles, so it is both a rhombus and a rectangle.

by "all" above, i mean 4.

this happens in general--you have "nicer" types of shapes (quadrilaterals can be pretty messy). Since the square is about the "nicest" four sided shape you can have, it is included in just about any category of "nice" four sided objects: rectangles, rhombuses, parallelograms, etc. Similarly, equilateral triangles (the nicest triangle) are also isosceles. There are sometimes definitions that stipulate "not-niceness." I believe a scalene triangle is not isosceles basically by definition. But not in the case of the square and the rhombus.

Yes. The condition for a rhombus is 4 equal sides. The condition for a square is 4 right angles and 4 equal sides; a square is just a more complex rhombus.

And is there a name for a 3-dimensional rhombus? Not a cube or a rectangular prism, but a conventional “diamond” shape in 3-D?

a rhomboid prism, perhaps? i’m not really sure, but that sounds about right to me.

a square and a rhombus are so different a square is like a box and a rhombus is like a kite a square has congurent sides and a rhombus has non i am in the 9th grade at south atlanta high school

In Europe Rhombus is a quadrilateral ( look like a square) , BUT !!!.... has NO STRAIGH angles. That ‘s why they call it a diamond, as a shape of the ring stone. In any posiciaon you will put it. In America, they ( i don’t no why) can put a square with straght angles on it angle, and call it Rhombus too. Well, it is wrong. What you will name a triangle, if you will draw it base up, or to the sides? Still a triangle. Right? So, the square in any position – still a square, not a diamond, as long as it has straight angles. In America they confuse children with their pictures of diamonds in shape of square in their Math books, starting from kindergarten. Unfortunately. Read here http://en.wikipedia.org/wiki/Rhombus

Every square is a rectangle, but not every rectangle is a square.
Every rectangle is a parallelogram, but not every parallelogram is a rectangle.
Every parallelogram is a rhombusbut not every rhombus is a parallelogram.
Every rhombus is a quadrilateral, but not every quadrilateral is a rhombus.
Every quadrilateral is a polygon, but….