a geometric surface consisting of one sheet, or of two sheets separated by a finite distance, whose sections parallel to the three coordinate planes are hyperbolas or ellipses. Equations x2/a2 + y2/b2 – z2/c2 = 1 (one sheet) or x2/a2 – y2/b2 – z2/c2 = 1 (two sheets) where a, b, and c are constants