(set theory, order theory, "on" a set A) A subset of the Cartesian product A×A (the set of ordered pairs (a, b) of elements of A).

(algebra) Any operation taking two operands.

(computing) A code that uses the binary digits (1 and 0), usually in groups of eight, to represent characters, machine instructions or other data.

(astronomy) A stellar system in which two stars orbit around their center of mass; double star.

(chemistry) A chemical compound composed of only two elements. / (chemistry) A compound composed of an element and a radical (a group of atoms that behave as one piece).

(arithmetic) arithmetic using binary numbers

(mathematics, computer science) A data structure in which each node has at most two children, each node but the root has one parent, and there are no cycles.

(biology) The process whereby a cell divides asexually to produce two daughter cells.

(mathematics) a function of the type f:V⨯V→V, or more generally, a function of the type f:U_1⨯U_2⨯...⨯U_n→V in which exactly two of the sets Uᵢ are equal to V.

(astronomy) A pair of quasars which gravitationally interact with each other, unlike a standard double quasar, which does not interact.