Symbol  Name  Description  Celestia EllipticalOrbit Parameter 

.  Apocenter  The location of the greatest distance between the orbiting body and the central body when the orbit is an ellipse. The apocenter is diametrically opposite the pericenter on the major axis of the orbit.  Not used 
lower case omega (ω), sometimes "w" 
Argument of Pericenter 
The angle from the ascending node to the pericenter, measured
in the plane of the orbit.
(Nobody knows why it's called "Argument". The earliest known use of the term "argument" to describe an angle was by Chaucer ~1391 in section 44 of his unfinished "A Treatise on the Astrolabe") 
ArgOfPericenter 

Eccentricity 
A measure of how elongated the orbit is.

Eccentricity 
.  Epoch  A significant time, often the time at which the orbital elements for an object are valid. An epoch is usually specified as a Julian date.  Epoch 

Inclination  The angle between this orbital plane and a reference plane.  Inclination 
.  Line of Apsides  The line between the Pericenter and the Apocenter.  Not used 

Line of Nodes  The line between the Ascending Node and the Descending Node  Not used 

Longitude of the Ascending Node  Angle from the origin of longitude of the reference plane to the orbit's ascending node: the point in its orbit where the orbiting body crosses the reference plane going "upward" or "northward". The time of this crossing is often used as the "elements' epoch."  AscendingNode 

Longitude of Pericenter 
Longitude of Ascending Node + Argument of Pericenter
This sometimes is called a "broken angle" since it includes angular measurements in two different planes. 
LongOfPericenter 

Mean Anomaly  The location of the body in the orbit: the product of mean motion and time since pericenter passage. I.e., the mean anomaly is 0.0 when the orbiting body is at pericenter, so defining the orbital elements at the epoch Tp (the time of the pericenter passage) eliminates the need to determine the mean anomaly.  MeanAnomaly 

Mean Longitude 
Mean Longitude is the longitude that an orbiting body would have
if its orbit were circular and its inclination were zero.
It is equal to the true longitude only at pericenter and apocenter.
L = M + = M + Ω + ω,
is the Longitude of Pericenter, Ω is the Longitude of the Ascending Node and ω is the Argument of Pericenter. 
MeanLongitude 

Mean Motion 
Average angular velocity needed to complete one orbit
See also: [in Web archives] http://www.tamuk.edu/math/scott/stars/docs/classical.pdf Note: n is measured in Radians, not degrees. 
Not used 

Pericenter Distance 
The shortest distance between the center of the orbiting body and
the center of the orbited body.

PericenterDistance 

Period 
The time to complete one orbit.

Period 

Semimajor Axis 
1/2 of the length of the orbit's major axis; half of (the pericenter distance
plus the apocenter distance)
a = ((m1+m2)*P^2)^(1/3) where
m1 and m2 are the masses of the orbiting bodies measured in units of the mass of the Sun, and P is the period of revolution about the barycenter measured in Years 
SemiMajorAxis 

Time of Pericenter Passage  A time at which the orbiting body passes through the pericenter, closest to the central body.  sometimes used as Epoch value 