Source : Webster's Revised Unabridged Dictionary (1913)

Sea onion \Sea" on"ion\ (Bot.)
   The officinal squill. See {Squill}.

Source : WordNet®

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