Bilateral means involving two sides or two parties. When two countries sign a bilateral agreement, they're making a deal between just the two of them, not with a whole group of nations. If your school arranges a bilateral exchange program with one school in Japan, students travel back and forth between those two schools specifically.

In science and medicine, bilateral often describes something affecting both sides of the body. Humans have bilateral symmetry, which means the left and right halves of our bodies mirror each other. A doctor might note that a patient has bilateral hearing loss, meaning both ears are affected.

The opposite is unilateral, meaning one-sided. If your friend makes a unilateral decision about what game to play without asking anyone else, they decided alone. But when two people make a bilateral decision, they work it out together, with both sides participating.