refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)--a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.

the mean distance between high and low tide.

synonymous with MEAN SEA LEVEL; an old–fashioned way of measuring land elevation and height by reference to the midpoint level of the sea.

the level of the ocean's surace, used as a standard in measuring land elevation or sea depths

The observed level of the sea surface relative to a predefined datum at any instant of time.

Fictional surface formed by the average height of the oceans, ignoring tidal cycles, weather, etc, and extended underneath the land to form a continous surface. This surface is not spherical. Commonly used approximations include "ellipsoids" (slightly-squashed spheres) and "geoids" (bumpier); the latter reflect variations in the strength of gravity in different locations. In many places the various kinds of "sea level" differ from each other by tens of meters, so next time you hear someone recite the altitude of Peak X down to the nearest centimeter, make sure you ask what "reference datum" they're using.

average height of the sea surface, usually measured over a period of years.

Sea level is height of a sea or ocean.

Height of the surface of the sea at any given time.

The surface elevation of the oceans at a given time. (See mean sea level).

The level of the world's oceans; all oceans are at sea level

Sea level is the normal level of the sea's surface, halfway between mean high and low tide levels.

The level of ocean waters. Municipalities are identified by their elevation above or below sea level. New Orleans, USA, for example, is 10 feet below sea level.

The ocean surface.

the level of the surface of the sea; a mid-point between the high and low tide

The average level of the oceans, used when finding water depths or land elevations.

The level of the sea after averaging out the short-term variations due to wind waves. It is used loosely as a synonym for mean sea level.

The height or level of the sea surface at any time. It is used as a reference for elevations above and below.

the surface of the sea, especially when halfway between mean high and low water. Mountains, plains, and ocean beds are measured as so many meters or feet above or below sea level. [AHDOS

Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level http://www.pol.ac.uk/psmsl/puscience/index.html#1, however, involves complex measurement, and accurately determining MSL can prove difficult.

Sea Level is the name of a group that mixed jazz, blues and rock that existed between 1976 and 1981.