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]