bis-

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

• prefix A form of bi-, sometimes used before s, c, or a vowel.

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

• prefix An occasional variant of bi- prefixed to roots beginning with a vowel, as in bisalternate
• prefix chemistry (obsolete) A variant of tri- used to indicate that the component that does not take the prefix occurs twice in the compound: a disoxide is of the form B2O
• prefix mathematics, chemistry A variant of bi-, used for substitutions of two complex rather than simple units.