Definition of "Case"

The basic principle governing case is:(20) The Case filter:A lexicalized NP must bear a Case feature in S-structure.‘Case’ with a capital C is here understood not as morphologically marked case, but as an abstract feature which will be present even in languages such as Swahili or Chinese which lack case marking on NPs (it is usually assumed however, that Case will be congruent with morphological case where the latter is present).

1988, Frederick J. Newmeyer, Linguistic Theory: Foundations

When we have clitic doubling constructions (with both a full NP and a clitic), the NP needs a dummy Case marker in order to get Case, as its “normal” Case is absorbed by the clitic, otherwise it will be ruled out by the Case Filter. It must be stressed that ‘Case’ here is abstract Case (written with capital C), a licensing requirement making arguments visible for θ-marking, and not morphological case.

1993, Anders Holmberg, Urpo Nikanne, Case and Other Functional Categories in Finnish Syntax