2. Adverb. (superlative of forte POS=adverb) ¹
3. Adverb. (music) The musical notation indicating that the piece is played louder than fortissimo. ¹
4. Noun. (music) The dynamic sign indicating that the piece should be played louder than fortissimo. Abbreviation: fff. ¹
¹ Source: wiktionary.com