It might also mean that given a function f(x), we signify (for example) the ROOTS of f with [tex]x^{*}[/tex]
Each root of f fulfills the equation:
[tex]f(x^{*})=0[/tex]
(In this case, therefore, [tex]x^{*}[/tex] is used as a symbol for an element of that subset R of the domain D of f which consists of the roots of f. The star distinguishes these x-values then from the rest of the x-values in D but not included in R.)
It is impossible to determine what is meant here without giving us the proper context.
EDIT:
Note that attachment of a star is a STANDARD notation for the complex conjugate (i.e, Matt Grime's suggestion); several authors don't bother to introduce a star symbol to denote specific roots (or elements of other subsets of the domain). However, some do, although it cannot be regarded as a standard or normative notation.
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