Source : Webster's Revised Unabridged Dictionary (1913)

Eelgrass \Eel"grass`\, n. (Bot.)
   A plant ({Zostera marina}), with very long and narrow leaves,
   growing abundantly in shallow bays along the North Atlantic
   coast.

Source : WordNet®

eelgrass
     n 1: submerged marine plant with very long narrow leaves found in
          abundance along North Atlantic coasts [syn: {grass wrack},
           {sea wrack}, {Zostera marina}]
     2: submerged aquatic plant with ribbonlike leaves; Old World
        and Australia [syn: {tape grass}, {wild celery}, {Vallisneria
        spiralis}]