The Collaborative International Dictionary of English v.0.48:

Longan \Lon"gan\, n. (Bot.)
   A pulpy fruit related to the litchi, and produced by an
   evergreen East Indian longanberry (Nephelium Longan).
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

longanberry \longanberry\ n.
   A tree (Dimocarpus longan) of Southeastern Asia to
   Australia grown primarily for its sweet translucent-fleshed
   edible fruit (the longan) resembling litchi nuts; -- it is
   sometimes placed in genera Euphoria or Nephelium.

   Syn: longan, lungen, Dimocarpus longan, Euphoria litchi,
        Nephelium longana.
        [WordNet 1.5]

   2. Asian fruit similar to litchi.

   Syn: dragon's eye.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

longan
    n 1: tree of southeastern Asia to Australia grown primarily for
         its sweet edible fruit resembling litchi nuts; sometimes
         placed in genera Euphorbia or Nephelium [syn: longan,
         lungen, longanberry, Dimocarpus longan, Euphorbia
         litchi, Nephelium longana]