CONOID, in geometry, any surface traced by a conic section rotating round either of its axes. Hence there are more varieties of conoids than of conics. These include (I) the ellipsoid, which may be (a) a prolate spheroid, shaped like a lemon, formed when an ellipse rotates round its major axis, or (b) an oblate spheroid, shaped like an orange or the earth, formed when the rotation is round the minor axis; a special limiting case is (c) the sphere, formed when the ellipse becomes a circle, all diameters then being equal. Archimedes (287-212 B.e.) preferred the term spheroid to ellipsoid, as seen in his work on conoids and spheroids. (2) The paraboloid generated by a parabola rotating about its axis, rounded near the vertex but spreading out indefinitely more and more like a cylinder. All rays emerging from the focus would be reflected from the paraboloid, considered as a mirror, these rays being par allel to the axis, as one immense beam—whence the construction of such reflectors approximately paraboloidal. (3) The hyper boloid, either of one sheet (nappe), by rotation round the conju gate axis—funnel-like, spreading out from a narrow neck, asymp totic outside to the "asymptote-cone"; or of two symmetric con gruent sheets, asymptotic inside to an "asymptote-cone," formed when rotation is round the transverse axis. (See ELLIPSOID,