DMDD is a DSM-5-TR diagnosis for children and teenagers with frequent, severe temper outbursts and an irritable or angry mood most days between them.
It was added in DSM-5 in 2013. It can only be first diagnosed between the ages of 6 and 18, with signs starting before age 10. ICD-11 does not list it as a separate diagnosis. It describes the same pattern under oppositional defiant disorder with chronic irritability and anger.
It was introduced partly to reduce the number of children diagnosed with bipolar disorder when their irritability was constant rather than coming in episodes.
It is not an adult diagnosis. Many adults with ADHD describe emotions that start quickly and feel very strong, which is a different thing and part of how ADHD presents for a lot of people. If you were given this diagnosis as a child, include it in your history for an adult assessment.