The term above mean sea level (AMSL) refers to the elevation (on the ground) or altitude (in the air) of any object, relative to the average sea level datum. AMSL is used extensively in radio (both in broadcasting and other telecommunications uses) by engineers to determine the coverage area a station will be able to reach. It is also used in aviation, where most heights are recorded and reported with respect to AMSL (see flight level), and in the atmospheric sciences.

The concept of a "mean sea level" is in itself rather artificial, because it is not possible to determine a figure for mean sea level for the entire planet, and it varies quite a lot even on a much smaller scale. This is because the sea is in constant motion, affected by the high and low pressure zones above it, the tides, local gravitational differences, and so forth. The best one can do is to pick a spot and calculate the mean sea level at that point and use it as a datum. For example, the Ordnance Survey uses a height datum based on the measurements of mean sea level at a particular gauge at Newlyn, Cornwall from 1915 to 1921 for their maps of Great Britain, and this datum is actually some 80 cm different from the mean sea level reading obtained on the other side of the country. An alternative is to base height measurements on an ellipsoid of the entire earth, which is what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is increasingly used to define mean sea level. Another alternative is to use a geoid based datum such as NAVD88.

Mean sea level (MSL) is a measure of the average height of the ocean's surface (such as the halfway point between the mean high tide and the mean low tide); used as a standard in reckoning land elevation. MSL also plays an extremely important role in aviation, where standard sea level pressure is used as the measurement datum of altitude at flight levels.

To an operator of a tide gauge, MSL means the "still water level"—the level of the sea with motions such as wind waves averaged out—averaged over a period of time such that changes in sea level, e.g., due to the tides, also get averaged out. One measures the values of MSL in respect to the land. Hence a change in MSL can result from a real change in sea level, or from a change in the height of the land on which the tide gauge operates.

In the UK, the Ordnance Datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921 the datum was MSL at the Victoria Dock, Liverpool.

In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level. It is used for a part of continental Europe and main part of Africa as official sea level.

Thomas Robert "Tommy" Black (born 26 November 1979) is an English footballer who plays for AFC Hornchurch.

Black began his career with Arsenal as a trainee, where he played alongside his older brother Michael. He was a regular in Arsenal's 1998 FA Premier Youth League-winning side, and was later sent out on loan twice for one month with Carlisle United and for a month with Bristol City. He made just two first team appearances for Arsenal, one in the League Cup against Middlesbrough on 30 November 1999 which Arsenal lost on penalties, and the other in the Premier League in a 1–0 win against Everton on 29 April 2000.

Unable to break into the Arsenal first team, Black signed for Crystal Palace in July 2000 for £500,000. Black played regularly and was well liked at Palace, receiving the Division One Player of the Month award in December 2002. After Palace were promoted a loan bid came in from Leeds United for both Black and Dougie Freedman - both were rejected. However, he did join Sheffield United on loan for a month, in December 2004, and in January 2006, still unable to break into the first team, he went out on loan again, this time to Gillingham, where he scored five goals and was an instant hit with the fans.