Hypo-, [Gr. under, beneath; akin to L. sub. See Sub-.].
A prefix signifying a less quantity, or a low state or degree, of that denoted by the word with which it is joined, or position under or beneath. [1913 Webster]
A prefix denoting that the element to the name of which it is prefixed enters with a low valence, or in a low state of oxidization, usually the lowest, into the compounds indicated; as, hyposulphurous acid. [1913 Webster]