Difference between UPSERT and MERGE?

From the PostgreSQL wiki, > MERGE is typically used to merge two tables, and was introduced in the 2003 SQL standard. The REPLACE statement (a MySQL extension) or *UPSERT* sequence attempts an UPDATE, or on failure, INSERT. This is similar to UPDATE, then for unmatched rows, INSERT. Whether concurrent access allows modifications which could cause row loss is implementation independent. Further PostgreSQL's [INSERT ... ON CONFLICT DO NOTHING/UPDATE](https://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_9.5#INSERT_..._ON_CONFLICT_DO_NOTHING.2FUPDATE_.28.22UPSERT.22.29) is marketed as *UPSERT* and was added in 9.5 What then is MERGE? And how does it fit into the mix?