A Dictionary of Music and Musicians/Piano
PIANO, 'soft.' This word, expressed in general by its initial p, is used to denote the least degree of strength except pianissimo. It is used, as is the case with most other directions, in full only when it is necessary to draw particular attention to its presence, or where it is unlikely that it should stand; for instance, in the Finale of Beethoven's PF. Sonata, Op. 2, No. 1, where the second subject is labelled 'Sempre piano e dolce.' Mezzo piano (abbreviated mp) denotes a degree of force slightly louder than piano. Beethoven was very fond of using a 'sudden piano' as a kind of surprise directly after a forte or fortissimo. Examples are very common throughout his works, and the occurrence of the sudden change only recently observed, and even now not always attended to makes a material difference in the performance of his works. [See Forte; vol. i. p. 556 a.]
[ J. A. F. M. ]