bi- Also bin- and bis-.
Two; having two; doubly.
[Latin, bi-, doubly, having two; bini, two together; bis, twice.]
To bisect a thing (Latin secare, to cut) is to divide it into two parts; a person who is bisexual is sexually attracted to both men and women; a bipedal animal uses only two legs for walking; a biplane has two pairs of wings, one above the other; a system or circuit that is bistable has two stable states; a binomial (Greek nomos, part or portion) is an algebraic expression of the sum or difference of two terms. In names of chemical compounds, bi- indicates the presence of two atoms or groups of a given kind: bicarbonate, bisulphite.
A biennial event takes place every other year; a plant is so described if it takes two years to grow from seed to fruition and die; something biannual occurs twice a year. Other time terms in bi- are ambiguous (biyearly, either every two years or twice a year; bimonthly, twice a month or every two months; biweekly, every two weeks or twice a week).
Examples using bin- include binocular (Latin oculus, eye), adapted for or using both eyes; binaural, involving both ears; binary, expressed in a system of numerical notation that has 2 rather than 10 as a base.
Bis- is used in the names of chemical compounds containing two secondary groups identically substituted or coordinated, as in bisphenol A, an organic compound formed from acetone by adding two molecules of phenol. A biscuit is literally a twice-cooked thing (via French from Latin coquere, to cook), because biscuits were first baked and then dried out in a slow oven so that they would keep.