In astrodynamics specific relative angular momentum () of orbiting body () relative to central body () is the relative angular momentum of per unit mass. Specific relative angular momentum plays a pivotal role in definition of orbit equations.

Specific relative angular momentum ()is defined as cross product of position vector and velocity vector of :

where:

• is orbital position vector of orbiting body relative to central body,
• is orbital velocity vector of orbiting body relative to central body.

Under standard assumptions for a orbiting body in a trajectory around central body at any given time the vector is perpendicular to the osculating orbital plane defined by orbital position and velocity vectors.

The magnitude of is denoted as :

For an elliptical orbit, it is twice the area per unit time swept out, hence twice the area of the ellipse divided by the orbital period, hence , which is .

The units of are km²s^-1.