An envelope of a set of curves is a curve that is tangent to these curves.
Examples
are the following:
 an astroid is the envelope of
straight lines
 an astroid is the envelope
of coaxial ellipses for
which the sum of the major and minor axes is constant
 a cardioid is the envelope of
the chords of a circle
 a cardioid is the envelope of circles
 a catacaustic is the envelope of
reflected rays
 a deltoid is the envelope of the
socalled Simson lines
 a diacaustic is the envelope of
refracted rays
 the lemniscate
is the envelope of circles with their centers on a rectangular hyperbola
 the nephroid is the envelope of a set of
circles with centers on a base circle and tangent to a diameter (of the base
circle)
In fact, the envelope of the normal of a curve is the same as the evolute
of the curve.
