A musical symbol that approximates that of the double-sharp.

The double-sharp, in music terms, is a marker that only appears on notes not previously marked with an accidental (a key signature applied to a note is nullified). It causes the note to fall two half-steps higher musically, typically from a white key to its next highest white neighbor. For example:

Ex enharmonically equals F#

Dx with a D# in the key signature enharmonically equals E

Cx enharmonically equals D

Of course, one could technically have triple-sharps and so forth, but besides being sadistic (or masochistic, depending on who the musician is), it is typically unneeded, unless one uses a key signature like Bbb major (A major), which (except in mental exercises in a music theory class) simply do not happen in real music. And yes, double sharps can appear in key signatures.