The crossword solution for the term 'Duo in an Ellipse' is FOCI, which stands for 'FOCI of an Ellipse'. FOCI are two points inside an ellipse that are used to define the shape of the ellipse. The two FOCI of an ellipse are always equidistant from the center of the ellipse, and the sum of the distances from each focus to any point on the ellipse is constant. Therefore, the term 'Duo in an Ellipse' refers to the two FOCI of an ellipse.