A Dictionary of Music and Musicians/Leggiero
LEGGIERO(Ital., also Leggieramente), lightly. The word is usually applied to a rapid passage, and in pianoforte playing indicates an absence of pressure, the keys being struck with only sufficient force to produce the sound. Leggiero passages are usually, though not invariably, piano, and they may be either legato or staccato; if the former the fingers must move very freely and strike the keys with a considerable amount of percussion to ensure distinctness, but with the slightest possible amount of force. Examples of legato passages marked leggieramente are found in the 25th variation of Beethoven's Op. 120, and in the finale of Mendelssohn's Concerto in G minor (which also contains the unusual combination of forte with leggiero); and of staccato single notes and chords in the finale of Mendelssohn's Concerto in D minor.
On stringed instruments leggiero passages are as a rule played by diminishing the pressure of the bow upon the strings, but the word generally refers rather to the character of the movement than to any particular manner of bowing. The Scherzo of Beethoven's Quartet in E♭, Op. 74, is marked leggiermente, although it begins forte, and the same indication is given for the 2nd variation of the Andante in the Kreutzer Sonata, which is piano throughout.