Foci is the plural of 'focus'. It is pronounced "FOE - sigh".

Foci of an ellipse

An ellipse has two foci. The definition of an ellipse is "A curved line forming a closed loop, where the sum of the distances from two points (foci) to every point on the line is constant." The two foci always lie on the major axis of the ellipse.

Ellipse showing its foci

See the links below for animated demonstrations of these concepts.