semimajor axis

noun
semi·ma·jor axis | \ˌse-mē-ˈmā-jər-, ˌse-ˌmī-, -mi-\

Definition of semimajor axis 

: one half of the major axis of an ellipse (such as that formed by the orbit of a planet)

First Known Use of semimajor axis

1818, in the meaning defined above

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The first known use of semimajor axis was in 1818

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