The term core argument is often used loosely to refer to an argument of the verb that is not expressed by adpositions or less common cases. In practice, it generally refers to an argument that is expressed by nominative or accusative, ergative or absolutive case, bear no case-marking, and/or are indexed on the verb.
In more recent versions of Role and Reference Grammar (Van Valin & LaPolla 1997), core argument refers just to an argument (there are also oblique core arguments, and non-core arguments (peripheral arguments) do not exist anymore). See core argument (in RRG).
Maybe the term originated with Foley & Van Valin (1984), an older version of Role and Reference Grammar (see core).
- Foley, William A. & Van Valin, Jr., Robert D. 1984. Functional syntax and universal grammar. Cambridge: Cambridge University Press.
- Thompson, Sandra A. 1997. "Discourse motivations for the core-oblique distinction as a language universal." In Kamio, Akio (ed.) Directions in functional linguistics. Amsterdam: Benjamins, 59-82.
- Van Valin, Jr., Robert D. & LaPolla, Randy J. 1997. Syntax: Structure, meaning and function. Cambridge: Cambridge University Press.