What is a distribution group in Active Directory?

In Active Directory, a distribution group refers to any group that doesn’t have a security context, whether it’s mail-enabled or not. In contrast, in Exchange Online, all mail-enabled groups are referred to as distribution groups, whether they have a security context or not.

For More Information Please Refer: https://docs.microsoft.com/en-us/exchange/recipients-in-exchange-online/manage-distribution-groups/manage-distribution-groups