(It.: ‘forcing’, ‘compelling’; gerund of sforzare).
A strong Accent. Like the past participle of the same verb, sforzato, it is abbreviated sf or sfz; fz is an abbreviation of forzando (‘forcing’) or forzato (‘forced’). In Beethoven and most other 19th-century composers it is used for an accent within the prevailing dynamic and need not necessarily be very loud; but in the work of many 20th-century composers it is intended as an exceptional mark, irrespective of its context.
See also Dynamics and Tempo and expression marks.