**
Limacon
** -
A polar equation of the form
*r* = *a* + *b* sin(*θ*)
or
*r* = *a* + *b* cos(*θ*)
, where
*a*, *b*≠ 0
.

**
Logarithmic Spiral
** -
A polar equation of the form
*r* = *ab*
^{
θ
}
.

**
Orientation
** -
The direction of a plane curve as the parameter increases.

**
Parameter
** -
A third variable (often time) which determines the values of
*x*
and
*y*
in parametric equations.

**
Parametric Equations
** -
Two equations of the form
*x* = *f* (*t*)
and
*y* = *g*(*t*)
, which specify the location of a point according to the variable
*t*
.

**
Plane Curve
** -
The set of all points
(*f* (*t*), *g*(*t*))
, where
*x* = *f* (*t*)
and
*y* = *g*(*t*)
are parametric equations.

**
Polar Axis
** -
The ray whose endpoint is the pole and which is the initial side of any angle measure in the polar plane.

**
Polar Coordinate System
** -
The system in which a point in the plane is specified according to an ordered pair
(*r*, *θ*)
in which
*r*
is a length and
*θ*
is an angle. The length
*r*
refers to the distance from the point to a fixed origin, called the pole. The angle
*θ*
is the angle whose initial side is a fixed ray (the polar axis) and whose terminal side contains the point. Under these circumstances, the point
(*r*, *θ*)
is expressed in polar coordinates.

**
Pole
** -
The fixed point in the polar coordinate system from which every point is
*r*
units away.

**
Rectangular Coordinate System
** -
The coordinate system in which every point is specified by exactly one ordered pair
(*x*, *y*)
. Here
*x*
is the distance between the point and a fixed line (the
*y*
-axis) and
*y*
is the distance between the point and a line fixed perpendicular to the other line (this line is the
*x*
-axis). The perpendicular lines are the axes, and the point
(*x*, *y*)
is expressed in rectangular coordinates.

**
Rose Curve
** -
A polar equation of the form
*r* = *a* sin(*nθ*)
or
*r* = *a* cos(*nθ*)
, where
*n*
is an integer.

**
Spiral of Archimedes
** -
A polar equation of the form
*r* = *aθ* + *b*
.