Editing Cone (section)

== Further terminology <span class="anchor" id="Terminology"></span>==
The perimeter of the base of a cone is called the ''directrix'', and each of the line segments between the directrix and apex is a ''generatrix'' or ''generating line'' of the lateral surface.  (For the connection between this sense of the term ''directrix'' and the [[Directrix (conic section)|directrix]] of a conic section, see [[Dandelin spheres]].)

The ''base radius'' of a circular cone is the [[radius]] of its base; often this is simply called the radius of the cone. {{anchor|Aperture}}The ''aperture'' of a right circular cone is the maximum angle between two generatrix lines; if the generatrix makes an angle ''θ'' to the axis, the aperture is 2''θ''. In [[optics]], the angle ''θ'' is called the {{anchor |half-angle}}''half-angle'' of the cone, to distinguish it from the aperture.
[[File:Acta Eruditorum - I geometria, 1734 – BEIC 13446956.jpg|thumb|Illustration from ''Problemata mathematica...'' published in [[Acta Eruditorum]], 1734]]

[[File:Cut cone unparallel.JPG|thumb|left|A cone truncated by an inclined plane]]
A cone with a region including its apex cut off by a plane is called a ''truncated cone''; if the [[Truncation (geometry)|truncation]] plane is parallel to the cone's base, it is called a ''[[frustum]]''.<ref name=":1" />  An ''elliptical cone'' is a cone with an [[ellipse|elliptical]] base.<ref name=":1" />  A ''generalized cone'' is the surface created by the set of lines passing through a vertex and every point on a boundary (see [[Visual hull]]).