Sentence:  Polarizability can be imagined as a three-dimensional <WSD>ellipsoid</WSD> centred on the centre of the molecule, as shown in Figure 10.4. 
Meaning of the word marked with <WSD> tags:  A surface, all of whose cross sections are elliptic or circular (including the sphere), that generalises the ellipse and in Cartesian coordinates ("x, y, z") is a quadric with equation "x2/a2" + "y2/b2" + "z2/c2" = 0.