The Collaborative International Dictionary of English v.0.48:

Luncheon \Lunch"eon\, n. [Prov. E. luncheon, lunchion, lunshin,
   a large lump of food, fr. lunch. See Lunch.]
   [1913 Webster]
   1. A lump of food. [Prov. Eng.]
      [1913 Webster]

   2. A portion of food taken at any time except at a regular
      meal. [obsolescnet]
      [1913 Webster]

   3. A lunch, especially one organized by a group as a formal
      social gathering.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Luncheon \Lunch"eon\, v. i.
   To take luncheon. --Beaconsfield.
   [1913 Webster]

WordNet (r) 3.0 (2006):

luncheon
    n 1: a midday meal [syn: lunch, luncheon, tiffin,
         dejeuner]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "luncheon":
   TV dinner, alfresco meal, barbecue, breakfast, brunch,
   buffet supper, clambake, coffee break, cookout, diner, dinner,
   elevenses, fish fry, high tea, hot luncheon, lunch, mash,
   meat breakfast, petit dejeuner, picnic, supper, tea, tea break,
   teatime, tiffin, wiener roast, wienie roast