semimajor axis

[ sem-ee-mey-jer, sem-ahy-, sem-ee-, sem-ahy- ]
/ ˈsɛm iˈmeɪ dʒər, ˈsɛm aɪ-, ˌsɛm i-, ˌsɛm aɪ- /

noun

Geometry. one half the major axis of an ellipse.
Astronomy. one half the major axis of the ellipse that one celestial body describes around another, as a planet around the sun or a satellite around a planet, equivalent to the mean distance between the two bodies.

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Origin of semimajor axis

First recorded in 1925–30; semi- + major
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020