What does ellipsoid mean?
An ellipsoid is a closed three-dimensional geometric surface whose plane cross-sections are all ellipses or circles, making it essentially a sphere that has been stretched or compressed along one or more axes. The term functions chiefly as a noun in mathematics, physics, astronomy, and geodesy, though it also serves adjectivally to describe anything shaped like such a figure. Ellipsoids come in characteristic varieties: an oblate ellipsoid is flattened like a mandarin orange, while a prolate ellipsoid is elongated like a rugby ball. The word matters well beyond pure geometry — Earth itself is best modelled not as a sphere but as an oblate ellipsoid, and global positioning systems rely on precisely defined reference ellipsoids to convert coordinates into locations on the ground. Derived forms include the adjective 'ellipsoidal'. While technical in register, the concept is intuitive: any smooth, egg- or ball-like solid without edges is likely to be an ellipsoid.
A solid geometric figure of which every plane section is an ellipse or circle; a three-dimensional analogue of the ellipse.
"M&Ms are roughly oblate ellipsoids rather than true spheres."
In geodesy and astronomy, qualified forms such as 'oblate ellipsoid' or 'prolate ellipsoid' specify whether the figure is flattened or elongated relative to a sphere.
Having the shape or form of an ellipsoid.
"The bacterium has small, ellipsoid spores."
In attributive use, the adjective 'ellipsoidal' is generally preferred over 'ellipsoid'.
Regular English plural, used whenever referring to more than one ellipsoid figure.
"The crystals under the microscope appeared as tiny ellipsoids of varying sizes."
Your GPS works because geodesists model the Earth not as a sphere but as a carefully chosen ellipsoid — get that shape slightly wrong and your location drifts by hundreds of metres.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of ellipsoid
The word 'ellipsoid' was formed in the 19th century from 'ellipse' plus the Greek-derived suffix '-oid', meaning 'resembling' or 'having the form of'. 'Ellipse' itself traces back through Latin to the Greek 'elleipsis' ('a falling short'), a term associated with Apollonius of Perga's classical work on conic sections. Related words sharing this Greek root include 'ellipsis', 'elliptical', and the sibling solids 'paraboloid' and 'hyperboloid'.
Related word forms
How ellipsoid is actually used
Primarily a technical term used in mathematics, physics, astronomy, and geodesy; it appears rarely in casual conversation. In scientific writing, 'ellipsoid' often carries precise qualifiers (e.g. oblate, prolate) indicating how it deviates from a perfect sphere. As a noun it takes the regular plural 'ellipsoids'; the adjective form 'ellipsoidal' is more common when describing shapes.
Easily confused with ellipsoid
An ellipse is a two-dimensional oval curve, whereas an ellipsoid is the corresponding three-dimensional solid.