A core argument is an °A-argument, a °P-argument, or an °S-argument, or another argument that is °coded like one of these.
Core arguments contrast with oblique nominals, defined here as nominals flagged by a flag other than nominative, accusative, ergative and absolutive. The term "core argument" has been current in typological syntax since the 1980s. ❡
Note that the "core" constituent in Role and Reference Grammar may also contain oblique arguments, so that in this context, one needs to talk about "direct (= non-oblique) core arguments" (e.g. Van Valin 2023: 81).
core arguments (INF) = the most salient arguments associated with a predication. Example: in Sally threw the letter into the wastebasket, Sally and the letter are construed as the most salient arguments; in The letter was thrown into the wastebasket, only the letter is construed as a salient argument. When there are two core arguments, the more salient argument is the subject and the less salient argument is the object. Core arguments are expressed by core argument phrases. (Section 6.1.1)