In music it’s a sharp, not a hashtag. Symbol: #. It’s a note (any note) plus a semitone. For instance, G sharp is note G plus a semitone. On a piano keyboard, it’s the black key located to the G’s right. On a guitar’s fretboard, it’s a fret closer to the guitar’s body. Also called diesis (deriving from Greek), or dièse (deriving from French).