geoid

[jee-oid]
noun
1.
an imaginary surface that coincides with mean sea level in the ocean and its extension through the continents.
2.
the geometric figure formed by this surface, an ellipsoid flattened at the poles.

Origin:
1880–85; < Greek geoeidḗs earthlike. See geo-, -oid

geoidal, adjective
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Collins
World English Dictionary
geoid (ˈdʒiːɔɪd)
 
n
1.  a hypothetical surface that corresponds to mean sea level and extends at the same level under the continents
2.  the shape of the earth

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
geoid   (jē'oid')  Pronunciation Key 
The hypothetical surface of the Earth that coincides everywhere with mean sea level and is perpendicular, at every point, to the direction of gravity. The geoid is used as a reference surface for astronomical measurements and for the accurate measurement of elevations on the Earth's surface.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

geoid

model of the figure of the Earth-i.e., of the planet's size and shape-that coincides with mean sea level over the oceans and continues in continental areas as an imaginary sea-level surface defined by spirit level. It serves as a reference surface from which topographic heights and ocean depths are measured. The scientific discipline concerned with the precise figure of the Earth and its determination and significance is known as geodesy.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Since the geoid is so irregular, geoid undulations cannot be computed directly but must be determined point by point.
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