Source : Webster's Revised Unabridged Dictionary (1913)

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.

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