To refrain from becoming angered, enraged, or upset due to some provocation; to maintain control of one's composure despite being angry or upset.

From Longman Dictionary of Contemporary English keep your temper to stay calm when it would be easy to get angry I was finding it increasingly difficult to keep my temper. → temper Examples from the Corpus keep your temper • When Felipe was grown up my father was wary and kept his temper.

Learning healthy coping skills to manage your anger and temper allows you to gain control over your behaviors in response to uncomfortable emotions. When you don't have control over how you act, it can make you more vulnerable to unhealthy relationships and put you in risky or dangerous situations.

lose/keep your temper (with somebody) to fail/manage to control your anger