What a "base" use case is?

Base use case is also known as including use case in unified modeling language (UML). A use case include the functionality that basically describe in another use case.

When two and more than two use case in the UML contain common properties then, it can be extracted to separate the use case.

Base use case is the use case from where the properties are copied into the smaller use case.

Each base use case represented as a sequence of the simple case in steps. By using the UML relationship small use case are added back in the base use case.