Datum
The earth’s mean radius is $r_{mean}=6370997.0m$. Because of the earth’s rotation the radius is slightly larger at the equator than at the poles. So the earth does not resemble a sphere, but more naturally an ellipsoid. What is more, the earth has a couple of large-scale bumps and dents due to gravitational hot spots, so that it is not even symmetric with respect to its rotation axis.
For that reason, the shape of the earth (more precisely the shape of the sea-level surface), is approximated by an ellipsoid. To account for asymmetries, an offset vector is used to dislocate the ellipsoid relative to the earth’s barycenter. The two properties together make up the so called datum.
The most commonly used datum is the WGS84 datum, that has been established as today’s most accurate world-wide standard since 1984.
For the WGS84 datum, the offset vector is zero, so it is often mistakenly identified with the corresponding ellipsoid, but a datum generally is not an ellipsoid. It is an ellipsoid shape plus an ellipsoid offset vector, which usually is not zero, except for the WGS84 datum.