Axis  -  The line over which a parabola is symmetric.

Branch  -  The term for each of the two distinct sections of the graph of a hyperbola.

Center  -  For an ellipse and hyperbola, the midpoint between the foci. For a circle, the fixed point from which all points on the circle are equidistant.

Circle  -  The set of all points equidistant from a given fixed point.

Conic  -  The intersection of a plane and a right circular cone.

Conjugate Axis  -  The line segment related to a hyperbola of length 2b whose midpoint is the center.

Degenerate Conic  -  A conic which is not a parabola, ellipse, circle, or hyperbola. These include lines, intersecting lines, and points.

Diameter  -  A line segment that contains the center of a circle whose endpoints are both on the circle, or sometimes, the length of that segment.

Directrix  -  For a parabola, it is the line whose distance from any point on the parabola is the same as the distance from that point to the focus. For a conic defined in polar terms, it is the line whose distance from any point on the conic makes a constant ratio with the distance between that point and the focus.

Eccentricity  -  The ratio

in an ellipse or hyperbola. Under the polar definition of conics, e is the constant ratio of the distance from a point to the focus and the distance from that point to the directrix.

Ellipse  -  The set of all points such that the sum of the distances from the point to each of two fixed points is constant.

Focus  -  For a parabola, the point whose distance from any point on the parabola is the same as the distance between that point and the directrix. For an ellipse, one of two points--the sum of whose distances to a point on the ellipse is constant. For a hyperbola, one of two points--the difference of whose distances to a point on the hyperbola is constant. Under the polar definition of a conic, it is the point whose distance from a point on the conic makes a constant ratio with the distance between that point and the directrix.

Hyperbola  -  The set of all points such that the difference of the distances between each of two fixed points and any point on the hyperbola is constant.

Major Axis  -  The line segment containing the foci of an ellipse whose endpoints are the vertices whose length is 2a.

Minor Axis  -  The line segment containing the center of an ellipse perpendicular to the major axis whose length is 2b.

Parabola  -  The set of all points such that the distance between a point on the parabola and a fixed line is the same as the distance between a point on the parabola and a fixed point.

Radius  -  A segment between the center of a circle and a point on the circle, or sometimes, the length of that segment.

Transverse Axis  -  The line segment that contains the center and whose endpoints are the two vertices of a hyperbola.

Vertex  -  (Plural = "vertices") For a parabola, the point halfway between the focus and the directrix. For an ellipse, one of two points where the line that contains the foci intersects the ellipse. For a hyperbola, one of two points at which the line containing the foci intersects the hyperbola.