Rod Burns wrote in sci.geo.geology:

I think the reference is not a point but a surface, the geoid surface,
which is an oblate spheroid that is determined I think by measuring
the gravity (I don't know how, thpough). This gives 0 meter altitude.


Very close. The ellipsoid is a mathematical reference surface used by
geodesists and cartographers. There are several, but GRS80 or WGS84 are
most commonly used today.

The geoid is a gravitational equipotential surface undulating above and
below the ellipsoid. The WGS84 value for the gravitational potential was
chosen for best fit to the global sea surface, but it doesn't follow the
sea surface. [In fact "the geoid" is a misnomer, since there are actually
several different potentials that have merit.]

Mean sea level is location dependent and may vary from the geoid due to
ocean dynamics. Different datums specify mean sea level by averaging
designated tide guages.

Elevation is measured with reference to a datum.

--
Jim Irwin
http://www.holoscenes.com