A Dictionary of Music and Musicians/Ossia
OSSIA, OPPURE, OVVERO. These words (the meaning of which is respectively 'Or it may be,' 'Or besides,' 'Or else') are used indifferently to mark a passage, generally printed above the treble or below the bass, which may be substituted for that written in the body or text of the work, being in most cases an easier version of the same kind of effect. For instance, 'ossia' is so used by Beethoven in the first movement of the Pianoforte Concerto in E♭ op. 73, 21 bars from the end. The same direction also occurs frequently in the pianoforte works of Schumann, Chopin, and Brahms. Liszt sometimes gives the easier passage in the text, and writes the more difficult one over it. These words were also used when the compass of the piano was in process of alteration; thus Moscheles sometimes adapts passages originally written for a full-sized piano, to the smaller compass, writing the passage for the smaller piano above that of the full-sized one.
The same object is attained by the words Plus facile or leichter.