Which musical term instructs a player to perform very quietly?

Italian dominates musical direction because printed music spread across Europe from Italian presses in the seventeenth and eighteenth centuries, and the vocabulary stuck. Piano means soft and forte means loud; adding the ending issimo makes a superlative, so pianissimo is very soft and fortissimo very loud. The markings appear as letters under the stave, p and pp, with f and ff at the other end, and composers extend the scale to ppp or fff when they want more. These are relative rather than absolute instructions. There is no decibel value attached, and a pianissimo in a full orchestra is louder than a mezzo-forte on a solo lute; the marking tells the player where to sit within the piece's own range and in balance with everyone else. The instrument known as the piano got its name from the same root, being originally the pianoforte, the keyboard that could play both soft and loud depending on touch.

The harpsichord could not vary volume by touch at all, which is precisely the limitation the piano was invented to solve.