A Dictionary of Music and Musicians/Forte
FORTE, loud: an Italian word, usually abbreviated into f. A lesser degree of loudness is expressed by mf—mezzoforte; a greater one by più f and ff, and the greatest of all by fff—fortissimo, as in Beethoven's 7th Symphony (Finale), 8th ditto (ist movement), Overture, op. 115 (at end), Leonore, No. 2 (8vo score, pp. 40, 76), or at the grand climax near the close of the Finale of Schubert's Symphony in C, at the end of the extraordinary long crescendo. ffff has been occasionally used by later composers, as in the Overture to 'Charlotte Corday,' by Benoit.
Fortepiano—afterwards changed to Pianoforte— was the natural Italian name for the new instrument which could give both loud and soft sounds, instead of loud only, as was the case with the harpsichord.
fp, is a characteristic sign in Beethoven, and one which he often uses; it denotes a sudden forte and an equally sudden piano. He will require it in the space of a single crotchet or even quaver, as in the Overture to Leonore, No. 2 (8vo score, pp. 31, 43, 51—fpp). Again, he was very fond of a forte passage succeeded suddenly, without any diminuendo, by a p, as in bars 64 to 66 of the Allegro of the same work, where the sudden p on the F♯ is miraculous; or in the reprise of the subject after the trumpet fanfares, where if the p is not observed the flute solo is overwhelmed. In a fine performance of his works half the battle lies in the exact observance of these nuances. No one marked them before him, and no one has excelled them since.