Is the keyword "ALIAS" actually used?

According to PostgreSQL 7.1 through 9.1 (now unsupported), ALIAS is listed as a reserved word, at least for SQL-99. Later versions do not show it - suggesting that it has been dropped as a reserved word. The old PostgreSQL docs do say *"the presence of a key word does not indicate the existence of a feature."* When aliasing a table or column I've seen AS, but never ALIAS. Where is (or was) the SQL keyword ALIAS used? Was it ever in-use or only ever reserved for future-use?