bis-

An occasional variant of bi- prefixed to roots beginning with a vowel, as in bisalternate

(chemistry, obsolete) A variant of di- used to indicate that the component that does not take the prefix occurs twice in the compound: a bisoxide is of the form B₂O (cf. dioxide for XO₂)

(mathematics, chemistry) A variant of di-, used for substitutions of two complex rather than simple units.