Webster's Revised Unabridged Dictionary · 1913

Sesquialteral

Sesquialteral , adjective

[Latin sesquialter once and a half; sesqui- + alter other: compare French sesquialtére.]

Once and a half times as great as another; having the ratio of one and a half to one.
Collocations (1)
Sesquialteral ratio (Mathematics) , the ratio of one and a half to one; thus, 9 and 6 are in a sesquialteral ratio.

Also: Sesquialterate