sea level

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The level of the ocean's surface, especially the level halfway between mean high and low tide.
• noun Mean sea level.

from The Century Dictionary.

• noun The surface of the sea, supposed to be level: commonly used as equivalent to mean sea-level, the level surface half-way between mean high and low water.

from the GNU version of the Collaborative International Dictionary of English.

• The level of the surface of the sea; any surface on the same level with the sea.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The nominal height of the surface of the oceans above which heights of geographical features and aircraft flight levels are measured.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun level of the ocean's surface (especially that halfway between mean high and low tide); used as a standard in reckoning land elevation or sea depth
• adjective lying below the normal level