double sharp

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A symbol (𝄪) placed before a note to indicate that it is to be raised by two semitones.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun music an accidental that indicates that a note should be played a whole step higher

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a musical notation of two sharps in front of a note indicating that it is to be raised by two semitones