A♯

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

• noun A musical note that lies a chromatic semitone above A and a diatonic semitone below B, thus being enharmonic to B♭; the eleventh semitone of the solfege; when calculated in equal temperament with a reference of A above middle C as 440 hertz, it has a frequency of approximately 466.164 Hz.