wrote:
What is it?

Thank you,
Christopher Lusardi

A fancy phrase for the path followed by some object (say, a
spacecraft) when the only force acting on it is gravity from
one other object (say, the Sun). This path is a conic section;
depending on the initial conditions, it may be a circle, ellipse,
parabola, or hyperbola.

-- Bill Owen

P.S. For you purists out there, yes, I do realize that the
second body must be spherically symmetric. :-)