The distance 
(sometimes also denoted 
)
from the focus to the conic
section directrix of a conic section. The following
table gives the focal parameter for the different types of conics, where 
is the semimajor axis, 
is the distances from the origin to the
focus, and 
is the eccentricity.
See also
Conic Section, Conic Section Directrix, Eccentricity, Focus, Semilatus Rectum, Universal Parabolic Constant
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Cite this as:
Weisstein, Eric W. "Focal Parameter." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/FocalParameter.html