One of the two points within an ellipse whose distance apart determines the eccentricity of the ellipse; pl.foci. The two foci lie on the major axis of the ellipse, at equal distances either side of its centre. In an elliptical orbit, one focus is occupied by the body being orbited; the other is referred to as the empty focus. If the orbit is a parabola or hyperbola, there is only one focus, occupied by the body being orbited.
Subjects: Astronomy and Astrophysics.