The Collaborative International Dictionary of English v.0.48:

Leek \Leek\ (l[=e]k), n. [AS. le['a]c; akin to D. look, G.
   lauch, OHG. louh, Icel. laukr, Sw. l["o]k, Dan l["o]g. Cf.
   Garlic.] (Bot.)
   A plant of the genus Allium (Allium Porrum), having
   broadly linear succulent leaves rising from a loose oblong
   cylindrical bulb. The flavor is stronger than that of the
   common onion.
   [1913 Webster]

   Wild leek, in America, a plant (Allium tricoccum) with a
      cluster of ovoid bulbs and large oblong elliptical leaves.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Allium porrum
    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]