ossia - passage of music, usually only a few bars long, presented as an alternative to the music written in the body of the score.
Ossias are usually used where the limitations of the instrument (eg Beethoven, who lived in a time when pianos were getting bigger, wrote ossias for performers who still had older pianos without some of the higher notes he used; or, when a piece written for one instrument provides alternative passages for those who want to play it on a different instrument) or of the ensemble (eg when a piece for a large orchestra, or including a rare instrument, may have to be played by a smaller orchestra, or without that special instrument), or of the performer (eg music written for young performers who haven't yet mastered the instrument, or who may have smaller hands, or music written for professionals but published for amateur domestic players) may preclude playing the notes as intended. Ossias may also in some styles of music be provided that are more complicated than the original, suggesting additional embellishments for more accomplished performers; I've also seen them in books for children of baroque music, offering suggestions for the ornamentation and variation that performers of the era were expected to improvise according to prevailing conventions. Apparently they're also used in scholarly reconstructions of music, to provide an alternative version found in a different manuscript, or a different interpretation of hard-to-read original notation.
I've always known what these were, but I've only recently learnt what they were called.
But the river tripped on her by and by, lapping
as though her heart was brook: Why, why, why! Weh, O weh
I'se so silly to be flowing but I no canna stay!