Instead, there are two focal points, called the foci, as you will see presently.
The villages of New England—the foci of blue laws and Puritanism.
The earth's orbit is an ellipse, one of the foci of which is occupied by the sun.
Each one is called a focus, and an ellipse can only have one pair of foci.
It has two foci, S and S', fixed points, by the aid of which we may trace ...