Sir is an honorific

Honorific

An honorific is a word or expression with connotations conveying esteem or respect when used in addressing or referring to a person. Sometimes, the term is used not quite correctly to refer to an honorary title...

 used as a title (see Knight

Knight

A knight was a member of a class of lower nobility in the High Middle Ages.By the Late Middle Ages, the rank had become associated with the ideals of chivalry, a code of conduct for the perfect courtly Christian warrior....

), or as a courtesy title

Courtesy title

A courtesy title is a form of address in systems of nobility used for children, former wives and other close relatives of a peer. These styles are used 'by courtesy' in the sense that the relatives do not themselves hold substantive titles...

 to address a man without using his given or family name

Family name

A family name is a type of surname and part of a person's name indicating the family to which the person belongs. The use of family names is widespread in cultures around the world...

 in many English speaking cultures. It is often used in formal correspondence (Dear Sir, Right Reverend Sir).

The term is often reserved for use only towards equals, one of superior rank

Command hierarchy

A command hierarchy is a group of people committed to carrying out orders "from the top", that is, of authority. It is part of a power structure: usually seen as the most vulnerable and also the most powerful part of it.-Sociology:...

 or status

Social status

In sociology or anthropology, social status is the honor or prestige attached to one's position in society . It may also refer to a rank or position that one holds in a group, for example son or daughter, playmate, pupil, etc....

, such as an educator or commanding officer

Commanding officer

The commanding officer is the officer in command of a military unit. Typically, the commanding officer has ultimate authority over the unit, and is usually given wide latitude to run the unit as he sees fit, within the bounds of military law...

, an elder (especially by a minor), or as a form of address from a merchant to a customer.

Equivalent terms of address are "ma'am" or "madam

Madam

Madam, or madame, is a polite title used for women which, in English, is the equivalent of Mrs. or Ms., and is often found abbreviated as "ma'am", and less frequently as "ma'm". It is derived from the French madame, which means "my lady", the feminine form of lord; the plural of ma dame in this...

" in most cases, or in the case of a very young woman, girl, or unmarried woman who prefers to be addressed as such, "miss".