WGS 84
WGS84 is short for the World Geodetic System as defined in 1984. It is the successor to the World Geodetic System of 1972.
The World Geodetic System defines an ellipsoid that is the most accurate description of the earth’s shape to date. With that description a place on earth can be geo-referenced with high accuracy.
The WGS84 ellipsoid has the following semi-minor and semi-major axis lengths: $r_{major}=6378137.0m$ and $r_{minor}=6356752.314245m$.
For WGS84, the two terms ellipsoid and datum are exchangable, but is has to be noted that this is not applicable in general.
Please also note that the WGS84 ellipsoid is practically identical to the GRS80 ellipsoid (Geodetic Reference System of 1980). Under practical conditions, the WGS84 datum is identical to the NAD83 datum (North American Datum of 1983).