What is foci?

Foci are points in a two-dimensional space that are used to describe the shape of an ellipse. An ellipse is a type of geometrical figure that looks like a squished circle. The foci of an ellipse are two points located on the major axis of the ellipse, which is the longest axis that cuts the ellipse into two equal halves. The distance between the foci and any point on the ellipse is constant; this distance is called the focal length. The foci are important because they determine the shape of the ellipse. When the two foci are close together, the ellipse is more circular, and when they are farther apart, the ellipse is more elongated. Foci also exist in other types of geometric figures, including hyperbolas and parabolas, where they play similar roles in determining the shape of these curves.