The Gentleman Usher of the Black Rod is more usually known simply as Black Rod. Black Rod is responsible for security in the House of Lords. He has a chair in the House of Lords and wears a distinctive black costume. Black Rod's role is similar to that of the Serjeant at Arms in the House of Commons.
Further information on this subject can be found from the following links.
About Parliament: Black Rod
Related glossary term: Serjeant at Arms (House of Commons)