As a musical direction: or rather; alternatively. Also as noun: an alternative version.
Usually standing alone, but occasionally as part of an explanatory phrase. It is used to indicate that an alternative version of a passage has been provided, frequently for ease of performance, but also in adapting music to an instrument with a different range, etc..
Origin
Late 19th century; earliest use found in John Stainer (1840–1901), musicologist and composer. From Italian ossia from o sia, lit. ‘or let-it-be’ from o or + sia, 3rd singular present subjunctive of essere to be.