sesqui-

from The Century Dictionary.

• noun A Latin prefix, meaning ‘One half more’—that is, an amount equal to one and a half times some unit, as in sesquitone; or an amount equal to a unit plus some part of itself, as in sesquialtera, sesquitertia, etc.

from the GNU version of the Collaborative International Dictionary of English.

• (Chem.) A combining form (also used adjectively) denoting that three atoms or equivalents of the substance to the name of which it is prefixed are combined with two of some other element or radical.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• prefix One and a half
• prefix In a ratio of 3 to 2