The eccentricity can also be interpreted as the fraction of the distance along the semimajor axis at which the focus
lies,

(1)

where
is the distance from the center of the conic section
to the focus.

The term "eccentricity" is also used in geodesy to refer to one of a number of similar quantities characterizing a spheroid. Given
a spheroid with equatorial radius and polar semi-axis , this (first) eccentricity, commonly denoted (Snyder 1987, p. 13; Karney 2023) but sometimes also
as (Beyer 1987, p. 131), is defined
as