An Etymological Dictionary of Astronomy and Astrophysics

In celestial mechanics, a combination of orbital elements
commonly used to distinguish between comets and asteroids. Objects whose Tisserand’s parameter value is smaller than 3 are considered to be dynamically cometary, and those with a value larger than 3 asteroidal. Also called Tisserand’s invariant.

See also: Named after François Félix Tisserand (1845-1896), French astronomer, Director of the Paris Observatory (1892).