Account and opportunity behaves as master detail in business logics but on UI it is a lookup relationship. Let me explain you a bit more.
You can create a Opportunity without filling account i.e it shows that there is a lookup relationship between account and Opportunity.
If you have created a opportunity with account and you delete that account then Opportunity will be deleted, this shows that it is in Master-Detail relationship. This is a standard behavior. So we can say it in both ways, but in the documentation it is a lookup relationship.
Account and opportunity behaves as master detail in business logics but on UI it is a lookup relationship. Let me explain you a bit more.
You can create a Opportunity without filling account i.e it shows that there is a lookup relationship between account and Opportunity.
If you have created a opportunity with account and you delete that account then Opportunity will be deleted, this shows that it is in Master-Detail relationship. This is a standard behavior.
So we can say it in both ways, but in the documentation it is a lookup relationship.
I hope it is clear now.
Best Regards,
Dayakar.D