The Collaborative International Dictionary of English v.0.48:

Scallion \Scal"lion\, n. [OF. escalone, eschaloingne, L. caepa
   Ascalonia onion of Ascalon; caepa onion + Ascalonius of
   Ascalon, fr. Ascalo Ascalon, a town in Palestine. Cf.
   Shallot.]
   1. (Bot.) A kind of small onion (Allium Ascalonicum),
      native of Palestine; the eschalot, or shallot.
      [1913 Webster]

   2. Any onion which does not "bottom out," but remains with a
      thick stem like a leek. --Amer. Cyc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

scallion
    n 1: plant having a large slender white bulb and flat
         overlapping dark green leaves; used in cooking; believed
         derived from the wild Allium ampeloprasum [syn: leek,
         scallion, Allium porrum]
    2: a young onion before the bulb has enlarged; eaten in salads
       [syn: green onion, spring onion, scallion]