Client Base

It's a fancy way to describe the set of clients that a company has.

The term applies to the customers who habitually buy from the company, and those that have a high likelihood of buying in the future.

If you run the local indie/foreign film movie theater, your client base might include the members of the film club at the local college. You might take this into account and offer a discount to people with a student ID.

Meanwhile, it probably doesn't make sense to offer a discount to people presenting a ticket stub from the recent WWE bout. These folks likely fall into your core client base.