Solana noob trying to learn how to develop here :)

I am reading through the documentation and working on a couple programs and keep seeing mentioning of a BaseAccount or Base Account in both programs and documentation, that being said, I don't see any clear definition of a base account in the developer documentation https://docs.solana.com/developing/programming-model/accounts

What exactly is a Base Account? Does every program have a Base Account? Or is it a global variable kind like the SystemProgram?