The Collaborative International Dictionary of English v.0.48:

Selah \Se"lah\, n. [Heb. sel[=a]h.] (Script.)
   A word of doubtful meaning, occuring frequently in the
   Psalms; by some, supposed to signify silence or a pause in
   the musical performance of the song.
   [1913 Webster]

         Beyond the fact that Selah is a musical term, we know
         absolutely nothing about it.             --Dr. W. Smith
                                                  (Bib. Dict.)
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Selah
   a word frequently found in the Book of Psalms, and also in Hab.
   3:9, 13, about seventy-four times in all in Scripture. Its
   meaning is doubtful. Some interpret it as meaning "silence" or
   "pause;" others, "end," "a louder strain," "piano," etc. The
   LXX. render the word by daplasma i.e., "a division."


Hitchcock's Bible Names Dictionary (late 1800's):

Selah, the end; a pause


U.S. Gazetteer Places (2000):

Selah, WA -- U.S. city in Washington
   Population (2000):    6310
   Housing Units (2000): 2408
   Land area (2000):     4.369241 sq. miles (11.316282 sq. km)
   Water area (2000):    0.057074 sq. miles (0.147820 sq. km)
   Total area (2000):    4.426315 sq. miles (11.464102 sq. km)
   FIPS code:            63280
   Located within:       Washington (WA), FIPS 53
   Location:             46.652109 N, 120.535134 W
   ZIP Codes (1990):     98942
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Selah, WA
    Selah