sesqui-

American Heritage® Dictionary of the English Language, Fourth Edition

1. One and a half: sesquicentennial.

Century Dictionary and Cyclopedia

1. n. 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. In chem., it is used to designate compounds in which there are one and a half times as many atoms or radicals of one member of the compound as of the other: thus, sesquioxid of iron is an oxid containing two atoms of iron to three of oxygen.

Wiktionary

1. One and a half
2. In a ratio of 3 to 2

GNU Webster's 1913

1. (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.