Regular or Irregular?

Hello girls, so this is the question. What you consider a regular period? The one who is every 28 days cycle?? Just that one? Every 29, 30 or even 21 days cycle are irregular for you?

The other day one of my cousins who have Hypothyroidism or hyper and irregular cycles, cyst and dont see the period every month,etc... So she told me while we where talking about it that she though irregular is a person who cycles arent just 28 days cycle even when the person dont have problems of cyst and see the period every month. To her you need to see you period 28 days to be regular. Im curious and hope for answer