Obligatory means required or expected by rules, customs, or social expectations. When something is obligatory, you have to do it, not because you want to, but because it's demanded or considered necessary.

At school, attendance is obligatory: you can't just skip class whenever you feel like it. Reading assigned chapters is obligatory homework. In sports, wearing safety equipment like helmets is often obligatory. These are clear requirements with real consequences if you ignore them.

But obligatory also describes things you feel socially pressed to do even when there's no official rule. You might give an obligatory thank-you wave when someone lets your family merge into traffic, or offer obligatory applause at the end of a school assembly. The obligatory birthday party invitation to your whole class means inviting everyone, even kids you barely know, because that's what's expected.

The word carries a hint of going through the motions rather than genuine enthusiasm. An obligatory smile is polite but not heartfelt. When adults call something “the obligatory office party,” they mean it's expected, not necessarily enjoyable. The opposite of obligatory is optional or voluntary: things you choose freely rather than feel required to do.