Active Directory
Also known as AD, AD DS
Microsoft's on-premises directory service, running on domain controllers and governing Windows domain authentication.
It has been the backbone of Windows networks for over two decades and remains in place at most businesses of any age, usually synchronized to Entra ID for cloud access.
It is also a favorite attacker target, because its design assumes an internal network is broadly trustworthy. Decades of accumulated permissions, stale accounts, and legacy protocol support give an intruder a great deal to work with once they have any domain foothold.
Where this comes up
Active Directory sits inside our identity and access work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.