What constitutes an "Active Member?"

This is a silly question, I suppose, but I've always been curious.

The forum statistics on the front page list new registrants, the number of threads, posts, total members, and active members.

While we have 24k registered members, the stat always hovers around 3-4k active members. How does vB determine what an active member is?

It's the number of users who have logged in since the past 60 days (the number of days cut-off is configurable).

Among other things, it prevents birthday announcements being clogged up with members who don't visit the forum anymore (inactive members aren't listed).

Cool. I figured there was a simple answer, though I would have assumed it something to do with members who have posted more than just members who have logged in.

Thanks blah~