Focus

The focus of a lens: The focus is the fixed point through which all the light rays coming from infinity pass.

For an ellipse, a special point on the semi-major axis: the sum (resp. the difference) of the distances from any point on an ellipse (resp. a hyperbola) to the foci is constant. The Sun is at the focus of the ellipses described by the planets orbiting around it (Kepler's 1st law).