To compel someone means to force them to do something, either through physical power, rules, or a powerful inner feeling they can't ignore. When a judge compels a witness to testify in court, the witness must answer questions whether they want to or not. When school rules compel students to arrive on time, those students have to follow the rule or face consequences.

Sometimes compel describes a force that isn't physical but feels just as strong. A compelling story is so interesting that you feel pulled to keep reading, even when you should be doing homework. When someone says “I felt compelled to help,” they mean something inside them made helping feel necessary, not optional. A friend's distress might compel you to comfort them. A fascinating question might compel a scientist to spend years searching for answers.

The word suggests a force stronger than ordinary persuasion. Your teacher might encourage you to join the science fair, but she can't compel you. However, a fire in the house would compel everyone inside to evacuate immediately. When you're compelled to do something, you don't feel like you have much choice in the matter, whether that force comes from outside rules or from something powerful inside yourself.