Noun
a musical notation of two sharps in front of a note indicating that it is to be raised by two semitones
Source: WordNetDouble-sharp
Adding a double accidental (double sharp) to F in this case only raisees F by one further semitone, creating G natural. Source: Internet
Conversely, adding a double sharp to any other note not sharped or flatted in the key signature raises the note by two semitones with respect to the chromatic scale. Source: Internet
For example, in the aforementioned key signature, any note that is not F, C, G, and D is raised by two semitones instead of one, so an A double sharp raises the note A natural to the enharmonic equivalent of B natural. Source: Internet
Likewise (and more commonly), a double sharp double sharp sign on a key signature with a single sharp indicates only a double sharp, not a triple sharp. Source: Internet