Source : Webster's Revised Unabridged Dictionary (1913)

Sesquialteral \Ses`qui*al"ter*al\, Sesquialterate
\Ses`qui*al"ter*ate\, a. [L. sesquialter once and a half;
   sesqui- + alter other: cf. F. sesquialt['e]re.]
   Once and a half times as great as another; having the ratio
   of one and a half to one.

   {Sesquialteral ratio} (Math.), the ratio of one and a half to
      one; thus, 9 and 6 are in a sesquialteral ratio.