directrix

noun
di·rec·trix | \-ˈrek-triks \
plural directrixes\-trik-səz \ also directrices\-trə-ˌsēz \

Definition of directrix 

1 archaic : directress

2 : a fixed curve with which a generatrix maintains a given relationship in generating a geometric figure specifically : a straight line the distance to which from any point of a conic section is in fixed ratio to the distance from the same point to a focus

First Known Use of directrix

1622, in the meaning defined at sense 1

History and Etymology for directrix

Medieval Latin, feminine of Late Latin director, from Latin dirigere

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The first known use of directrix was in 1622

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