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Cyc predicates that take exactly two arguments are sometimes called slots, and the string “slot” shows up in the names of quite a few collections of binary predicates such as #$ActorSlot, #$QuantitySlot, #$FunctionalSlot, and more.

This naming practice is a historical remnant of the earliest days of Cyc, during which it was a [frame-based] system. In those very early versions of Cyc, all predicates were binary, and they were called “slots”.