Bi-, comb. form (often bin- before a vowel) forming nouns and adjectives meaning:
1 having two; a thing having two (bilateral; binaural; biplane).
2 a occurring twice in every one or once in every two (bi-weekly). b lasting for two (biennial).
3 doubly; in two ways (biconcave).
4 Chem. a substance having a double proportion of the acid etc. indicated by the simple word (bicarbonate).
5 Bot. & Zool. (of division and subdivision) twice over (bipinnate).