Unilateral means done by one side or one person acting alone, without agreement or cooperation from others involved. When a country makes a unilateral decision, it acts on its own without consulting its allies or partners. When your friend makes a unilateral choice about which game to play at recess without asking anyone else's opinion, they're deciding for the whole group by themselves.

Think of it like a one-sided conversation where only one person talks, or a one-sided argument where only one viewpoint gets heard. In contracts and agreements, a unilateral action is when one party does something without the other's input or consent, which can cause problems because agreements usually require both sides to participate.

You might hear about a coach making a unilateral substitution without consulting the team captain, or a teacher making a unilateral decision to change the class schedule. The word often carries a hint of criticism because it suggests someone acted without considering others who should have been involved. When important decisions affect multiple people, unilateral actions can feel unfair or high-handed. The opposite would be bilateral (two-sided) or multilateral (many-sided), where everyone participates in the decision.