ellipse

[ih-lips]
noun Geometry.
a plane curve such that the sums of the distances of each point in its periphery from two fixed points, the foci, are equal. It is a conic section formed by the intersection of a right circular cone by a plane that cuts the axis and the surface of the cone. Typical equation: (x 2 / a 2 ) + (y 2 / b 2 ) = 1. If a = b the ellipse is a circle.

Origin:
1745–55; < French < Latin ellīpsis ellipsis; or by back formation from the plural ellipses

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World English Dictionary
ellipse (ɪˈlɪps)
 
n
a closed conic section shaped like a flattened circle and formed by an inclined plane that does not cut the base of the cone. Standard equation x²/a² + y²/b² = 1, where 2a and 2b are the lengths of the major and minor axes. Area: πab
 
[C18: back formation from ellipsis]

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Etymonline
Word Origin & History

ellipse
1656 (implied in ellpitical), from Fr. ellipse, from L. ellipsis "ellipse," also, "a falling short, deficit," from Gk. elleipsis (see ellipse), because the conic section of the cutting plane makes a smaller angle with the base than does the side of the cone, hence, a "falling
short." First applied by Apollonius of Perga (3c. B.C.E.).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
ellipse   (ĭ-lĭps')  Pronunciation Key 
A closed, symmetric curve shaped like an oval, which can be formed by intersecting a cone with a plane that is not parallel or perpendicular to the cone's base. The sum of the distances of any point on an ellipse from two fixed points (called the foci) remains constant no matter where the point is on the curve.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary
ellipse [(i-lips)]

In geometry, a curve traced out by a point that is required to move so that the sum of its distances from two fixed points (called foci) remains constant. If the foci are identical with each other, the ellipse is a circle; if the two foci are distinct from each other, the ellipse looks like a squashed or elongated circle.

Note: The orbits of the planets and of many comets are ellipses.
The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
The main axis of the planet's orbital ellipse shifts each time it goes round
  the sun.
If you were to cut a coelacanth across the middle, you'd see that it's almost
  an ellipse.
From that moment on, he never had any doubts about the ellipse and stated that
  as his first law of planetary motion.
He connected the whole landscape with the geometry of the circle and the
  ellipse.
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