Source : Webster's Revised Unabridged Dictionary (1913)

Sesquialter \Ses`qui*al"ter\, Sesquialtera \Ses`qui*al"ter*a\,
   n. [NL. sesquialtera.] (Mus.)
   A stop on the organ, containing several ranks of pipes which
   re["e]nforce some of the high harmonics of the ground tone,
   and make the sound more brilliant.