Amenorrhea is a medical term for missed periods. It is not the same as irregular periods. There are two types of amenorrhea. Primary amenorrhea is when a person older than 15 years does not experience menarche or periods and secondary amenorrhea is where a person misses 2-3 menstrual cycle.
Sources:
https://www.mayoclinic.org/diseases-conditions/amenorrhea/symptoms-causes/syc-20369299
https://www.webmd.com/infertility-and-reproduction/guide/absence-periods
https://my.clevelandclinic.org/health/diseases/3924-amenorrhea