[[A restrictive focus marker is a marker indicating that a proposition applies to a particular referent against the background of other possible referents and not to more.]]
Another term is "exclusive focus marker" (König 1991: Chapter 5)
restrictive (focus) operator (a.k.a. exclusive focus operator) (CXN) = a focus operator that indicates that the focus is the restricted information in a restricting counterpresuppositional contrast. Example: in the exchange John bought apples and peaches. No, he only bought PEACHES, only is a restrictive operator. (Section 11.4.1)