from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun The hypothetical surface of the earth that coincides everywhere with mean sea level.

from The Century Dictionary.

  • noun An imaginary surface which coincides with the mean sea-level over the ocean, and extends under the continents everywhere at that level at which the mean surface of the sea would stand if it were allowed to flow in through a small subterranean canal.
  • noun See geode.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun A surface of constant gravitational potential at zero elevation.


from The American Heritage® Dictionary of the English Language, 4th Edition

[German, from Greek geoeidēs, earthlike : , earth + -oeidēs, -oid.]



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  • See discussion on potato Earth.

    March 31, 2011

  • "There have been many definitions of the "geoid" over 150 years or so. Here is the one currently adopted at the National Geodetic Survey:

    geoid: The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level

    Even though we adopt a definition, that does not mean we are perfect in the realization of that definition. For example, altimetry is often used to define "mean sea level" in the oceans, but altimetry is not global (missing the near polar regions). As such, the fit between "global" mean sea level and the geoid is not entirely confirmable. Also, there may be non-periodic changes in sea level (like a persistent rise in sea level, for example). If so, then "mean sea level" changes in time, and therefore the geoid should also change in time. These are just a few examples of the difficulty in defining "the geoid"."

    --What is the geoid?

    March 31, 2011