The Collaborative International Dictionary of English v.0.48:

Sea onion \Sea" on"ion\ (Bot.)
   The officinal squill. See Squill.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Squill \Squill\ (skw[i^]l), n. [F. squille (also scille a
   squill, in sense 1), L. squilla, scilla, Gr. ?.]
   1. (Bot.)
      (a) A European bulbous liliaceous plant (Urginea
          maritima, formerly Scilla maritima), of acrid,
          expectorant, diuretic, and emetic properties, used in
          medicine. Called also sea onion.
      (b) Any bulbous plant of the genus Scilla; as, the
          bluebell squill (Scilla mutans).
          [1913 Webster]

   2. (Zool.)
      (a) A squilla.
      (b) A mantis.
          [1913 Webster]

WordNet (r) 3.0 (2006):

sea onion
    n 1: having dense spikes of small white flowers and yielding a
         bulb with medicinal properties [syn: sea squill, sea
         onion, squill, Urginea maritima]
    2: European scilla with small blue or purple flowers [syn:
       spring squill, Scilla verna, sea onion]