The latus rectum of a conic section is the chord through a focus parallel to the conic section directrix (Coxeter 1969). "Latus rectum" is a compound of the Latin latus, meaning "side," and rectum, meaning "straight."

Half the latus rectum is called the semilatus rectum.

Coxeter, H. S. M. Introduction to Geometry, 2nd ed. New York: Wiley, pp. 116-118, 1969.

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Weisstein, Eric W. "Latus Rectum." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/LatusRectum.html