The Collaborative International Dictionary of English v.0.48:

Glen \Glen\, n. [Of Celtic origin; cf. W. glyn a deep valley,
   Ir. & Gael. gleann valley, glen.]
   A secluded and narrow valley; a dale; a depression between
   hills.
   [1913 Webster]

         And wooes the widow's daughter of the glen. --Spenser.
   Glengarry

WordNet (r) 3.0 (2006):

glen
    n 1: a narrow secluded valley (in the mountains)

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "glen":
   bottom, bottom glade, bottoms, combe, dale, dell, dingle, gap,
   gill, grove, intervale, lunar rill, pass, ravine, strath, trench,
   trough, vale, valley, wadi



U.S. Gazetteer Places (2000):

Glen, MS -- U.S. town in Mississippi
   Population (2000):    286
   Housing Units (2000): 133
   Land area (2000):     4.679696 sq. miles (12.120357 sq. km)
   Water area (2000):    0.006543 sq. miles (0.016947 sq. km)
   Total area (2000):    4.686239 sq. miles (12.137304 sq. km)
   FIPS code:            27540
   Located within:       Mississippi (MS), FIPS 28
   Location:             34.859967 N, 88.418633 W
   ZIP Codes (1990):     38846
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Glen, MS
    Glen