Senza sordino
Without mute.
What it means
Senza sordino means literally 'without mute' and calls for removing a previously applied mute and continuing to play with the instrument's full, unmuted sound.
The instruction presupposes that con sordino ('with mute') was called for at an earlier point; senza sordino cancels that instruction.
In the notation
Senza sordino is notated as a text instruction above the staff, usually placed exactly at the point from which playing should resume without the mute.
Musical use
On string instruments, an applied mute audibly alters the timbre: it damps high overtones more strongly than low ones, making the sound softer, quieter, and more veiled. Senza sordino restores this original, brighter, fuller sound quality.
On brass instruments and on the piano (referring to the damper pedal), senza sordino is used analogously to indicate the lifting of a previously requested muting, though the sonic effect differs by instrument.
Senza sordino should be distinguished from con sordino, its direct counterpart, and from more general dynamic markings such as forte, which likewise produce a fuller sound but have nothing to do with physically removing a mute.
Performance practice
Remove the mute in good time before the marked point, so the transition doesn't stall. When switching frequently between con sordino and senza sordino, plan the exact timing of putting the mute on and taking it off carefully, especially when only a few bars are available for it.