Focus
FO'CUS, noun plural focuses, or foci. [Latin focus a fire, the hearth.]
1. In optics, a point in which any number of rays of light meet, after being reflected or refracted; as the focus of a lens.
2. In geometry and conic sections, a certain point in the parabola, ellipsis and hyperbola, where rays reflected from all parts of these curves, concur or meet.
The focus of an ellipsis, is a point towards each end of the longer axis, from which two right lines drawn to any point in the circumference, shall together be equal to the longer axis.
The focus of a parabola, is a point in the axis within the figure, and distant from the vertex by the fourth part of the parameter.
The focus of a hyperbola, is a point in the principal axis, within the opposite hyperbolas, from which if any two lines are drawn, meeting in either of the opposite hyperbolas, the difference will be equal to the principal axis.
3. A central point; point of concentration.