[dih-rek-triks, dahy-]
noun, plural directrixes, directrices [dih-rek-tri-seez, dahy-, dahy-rek-trahy-seez] .
Geometry. a fixed line used in the description of a curve or surface. See diag. under parabola.
Archaic. a directress.

1615–25; < Neo-Latin; see direct, -trix

See -trix. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
directrix (dɪˈrɛktrɪks, daɪ-)
1.  geometry a fixed reference line, situated on the convex side of a conic section, that is used when defining or calculating its eccentricity
2.  a directress
[C17: New Latin, feminine of director]

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
directrix   (dĭ-rěk'trĭks)  Pronunciation Key 
A straight line used in generating a curve such as a parabola.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Derive the equation of a parabola given a focus and directrix.
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