The following words referring to dynamics are in common use:
Pianisissimo (ppp)—as softly as possible. (It will be noted that this is a sort of hyper-superlative of piano.)
Pianissimo (pp)—very softly. (The superlative of piano.)
Piano (p)—softly.
Mezzo piano (mp)—medium softly.
Mezzo forte (mf)—medium loudly.
Forte (f)—loudly (lit. strong).
Fortissimo (ff)—very loudly. (The superlative of forte.)
Fortisissimo (fff)—as loudly as possible.
The lack of a one-word comparative degree in the case of both piano and forte seems to necessitate the hyper-superlative degree as given above, but the practice of using four, or even five p's or f's is not desirable.