Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. A foursided plane figure with opposite sides parallel.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A convex quadrilateral in which each pair of opposite edges are parallel and of equal length.
from the GNU version of the Collaborative International Dictionary of English
 n. A rightlined quadrilateral figure, whose opposite sides are parallel, and consequently equal;  sometimes restricted in popular usage to a rectangle, or quadrilateral figure which is longer than it is broad, and with right angles.
from The Century Dictionary and Cyclopedia
 n. In geometry, a quadrilateral whose opposite sides are parallel.
 n. A pantograph.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. a quadrilateral whose opposite sides are both parallel and equal in length
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
Examples

*This is reputed to be the first rock song to contain the word parallelogram in it.

A square only requires a peg at each corner, with a chalked string drawn from peg to peg; and an oblong, or parallelogram, is made by joining two common squares, and taking off the corners if required.
The Lady's Country Companion: or, How to Enjoy a Country Life Rationally

This addition rule is known as the parallelogram law.

This game is played by marking out in the dust or sand a parallelogram, which is subdivided into a varying number of compartments.

When opposite sides of the figure are parallel to each other it is termed a parallelogram, no matter what the angle of the adjoining lines in the figure may be.

She was particularly perplexed by geometry; she aroused our hilarity by always calling a parallelogram a parallelOgram, with a strong emphasis on the penultimate syllable; and she spent several days repeating over to herself, with a mystified countenance, the famous words, "The square of the hypothenuse is equal to the sum of the squares of the two legs."

This evening Mr. Spong come, and sat late with me, and first told me of the instrument called parallelogram,

When the diagonals of the original quadrilateral are congruent, then the parallelogram is a rhombus.

Either through guess and check or by an analysis of the situation, stu  dents should discover the following: When the diagonals of the original quadrilateral are perpendicular, the parallelogram is a rectangle.

Geometric Wonders 135 When the diagonals of the original quadrilateral are congruent and per  pendicular, then the parallelogram is a square.
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