I42
As PAXboy noted the earth is an oblate spheroid - but only just. 
WGS-84, used as the default spheroid by GPS, has an equatorial diameter of 12,756 km. The polar diameter is 43 km less - a difference of 0.3%.
WGS-84 actually treats the Earth as an ellipsoid (a body of rotation defined by an ellipse rotated about its minor axis). A more accurate model flattens the South pole end rather more, and makes the North pole end a bit pointier (hence, 'everything has gone pear shaped') but that make the spherical trig calculations far too difficult and the ellipsoid is good enough for government work.