What is "gentleman usher of the black rod"

Gentleman \Gen"tle*man\, n.; pl. Gentlemen. [OE. gentilman nobleman; gentil noble + man man; cf. F. gentilhomme.]

1. A man well born; one of good family; one above the condition of a yeoman.

2. One of gentle or refined manners; a well-bred man.

3. (Her.) One who bears arms, but has no title.

4. The servant of a man of rank.

   The count's gentleman, one Cesario.
   --Shak.

5. A man, irrespective of condition; -- used esp. in the plural (= citizens; people), in addressing men in popular assemblies, etc.

   Note: In Great Britain, the term gentleman is applied in a limited sense to those having coats of arms, but who are without a title, and, in this sense, gentlemen hold a middle rank between the nobility and yeomanry. In a more extended sense, it includes every man above the rank of yeoman, comprehending the nobility. In the United States, the term is applied to men of education and good breeding of every occupation.

   Gentleman commoner, one of the highest class of commoners at the University of Oxford.

   Gentleman usher, one who ushers visitors into the presence of a sovereign, etc.

   Gentleman usher of the black rod, an usher belonging to the Order of the Garter, whose chief duty is to serve as official messenger of the House of Lords.

   Gentlemen-at-arms, a band of forty gentlemen who attend the sovereign on state occasions; formerly called gentlemen pensioners. [Eng.]