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The Collaborative International Dictionary
Bis-

Bis- \Bis-\, pref. A form of Bi-, sometimes used before s, c, or a vowel.

Douglas Harper's Etymology Dictionary
bis-

word-forming element meaning "twice," from Latin bis "twice, in two ways, doubly," from Old Latin dvis, cognate with Sanskrit dvih, Avestan bish, Greek dis, Middle High German zwis "twice," from PIE *dwo- "two" (see two). Also the form of bi- used before -s-, -c-, or a vowel.

Wiktionary
bis-

pre. 1 An occasional variant of bi- prefixed to roots beginning with a vowel, as in ''bisalternate'' 2 (context chemistry obsolete English) A variant of tri- used to indicate that the component that does ''not'' take the prefix occurs twice in the compound: a ''bisoxide'' is of the form B2O 3 (context mathematics chemistry English) A variant of di-, used for substitutions of two complex rather than simple units.