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):

leek
    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: related to onions; white cylindrical bulb and flat dark-green
       leaves

Easton's 1897 Bible Dictionary:

Leek
   (Heb. hatsir; the Allium porrum), rendered "grass" in 1 Kings
   18:5, 2 Kings 19:26, Job 40:15, etc.; "herb" in Job 8:12; "hay"
   in Prov. 27:25, and Isa. 15:6; "leeks" only in Num. 11:5. This
   Hebrew word seems to denote in this last passage simply herbs,
   such as lettuce or savoury herbs cooked as kitchen vegetables,
   and not necessarily what are now called leeks. The leek was a
   favourite vegetable in Egypt, and is still largely cultivated
   there and in Palestine.