sea level

from WordNet (r) 3.0 (2006)

sea level
    n 1: 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

from The Collaborative International Dictionary of English v.0.48

Sea level \Sea" lev"el\
   The level of the surface of the sea; any surface on the same
   level with the sea.
   [1913 Webster] Sealgh

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