What is what does foci means?

Foci (plural of focus) refers to the two fixed points in an ellipse. In geometry, an ellipse is a curved shape that looks like a stretched out circle. The two foci of an ellipse are located on the major axis, which is the longest diameter of the ellipse. The sum of the distances from any point on the ellipse to the two foci is constant and equal to the length of the major axis. This property of the foci is what distinguishes an ellipse from other conic sections like circles and hyperbolas. Foci can also refer to the central points of a hyperbola, parabola or other curved shape.