semimajor axis

[sem-ee-mey-jer, sem-ahy-, sem-ee-, sem-ahy-]
  1. Geometry. one half the major axis of an ellipse.
  2. 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.

Origin of semimajor axis

First recorded in 1925–30; semi- + major Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018