Together, xvalues and lvalues are called “glvalues” (“generalized” lvalues). Rvalues, as currently known in the core language clauses, are renamed to “prvalues” (“pure” rvalues). Together, prvalues and xvalues are called “rvalues,” reflecting the use of that term in the library clauses and the intended uses in reference binding, overload resolution, and template argument deduction. […] An xvalue is the result of certain kinds of expressions involving rvalue references (8.3.2).
Source: wiktionary