The Collaborative International Dictionary of English v.0.48:

Zostera \Zos"te*ra\, n. [NL.] (Bot.)
   A genus of plants of the Naiadaceae, or Pondweed family.
   Zostera marina is commonly known as sea wrack, and
   eelgrass.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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