To become calm.

1. When you calm down, you become less active, nervous, or upset. When you calm other people down, you do something to make them less active, nervous, or upset. Settle down is similar to calm down.
2 When a confused or violent situation becomes less confused or violent, it calms down. When you calm a confused or violent situation down, you make it less confused or violent and more calm. Settle down is similar to calm down.