a parallelogram with four right angles
"The architect designed the room as a rectangle, ensuring every corner formed a perfect ninety-degree angle."
A quadrilateral having opposing sides parallel and four right angles.
"The garden plot is a rectangle because its opposite sides are parallel and all four corners form perfect right angles."
In plain English: A rectangle is a four-sided shape with opposite sides that are equal and parallel, where every corner forms a perfect right angle.
"The classroom has a rectangular whiteboard on the wall."
Usage: Use rectangle to describe any shape with four right angles, such as the screen of your phone or a piece of paper. Do not confuse it with square when referring specifically to shapes where all four sides are equal in length.
The word rectangle comes from the Medieval Latin term rectangulum, which literally means "having a right angle." It entered English through Middle French, combining the roots for "straight" or "correct" and "corner."