A rectangle is a quadrilateral parallelogram with 4 right angles. Two pairs of parallel line segments that intersect perpendicularly.
A square or an elongated square.

"Rectangle" is a somewhat troublesome word in standard American English. In geometry, the term means exactly what its linguist derivation would give: a figure with four right angles. The size of the sides is irrelevant, and the square is a subset of rectangles.

However, I am sure that I am not alone in remembering back to preschool, and being given the most simple puzzle of all: a piece of cardboard with four pieces to insert. A triangle, a circle, a rectangle, and a square. Lessons learned so early are hard to forget, and I usually use the term "rectangle" to mean an object whose sets of parallel lines are not equal to each other. For example, I might say

The Pentium Pro appears to be a rectangle, but that is an optical illusion, it is actually a square.
Which is technically incorrect, since squares are indeed retangles. Clarification of the issue tends to make the statement even more clumsy. Once again, our preschool system is responsible for American's backwardness in mathematics and science.

Rec"tan`gle (r?k"t??`g'l), n. [F., fr. L. rectus right + angulus angle. See Right, and Angle.] Geom.

A four-sided figure having only right angles; a right-angled parallelogram.

⇒ As the area of a rectangle is expressed by the product of its two dimensions, the term rectangle is sometimes used for product; as, the rectangle of a and b, that is, ab.

Rec"tan`gle, a.

Rectangular.

[R.]

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