The traditional name for the form of a noun when a member of this word class is used not just to name but to address someone or something is vocative. Together with the imperative of verbs, the vocative, strictly speaking, serves the so-called conative function. Thus the Indo-European languages (but not only) have to one or another extent maintained a vocative case and its concomitant separate desinence (ending) in the paradigm devoted to this naming or addressing function, although the overarching tendency in the history of these languages is for the vocative to fall together with the nominative in form. In a language like Russian, for instance, where the vocative overwhelmingly gave way to the nominative (except for the recent resurgence of the so-called “new vocative”), the form of the noun used for address is the same as the nominative, although Russian still has fossilized instances of the old vocative in religious terms like Боже (for nom. Бог ‘God’) and Господи (for nom. Господь ‘Lord’), which are now just part of common parlance as exclamations rather than terms of address.
Like any other language, English has a vocative intercalated in discourse that is identical in form with the nominative (subjective); moreover, as in all languages, English vocatives serve the phatic and emotive functions over and above the conative. A word like sir in military practice, for example, is a token of deference and is de rigueur in speech whenever a person of higher rank is addressed. This sort of practice can be called the “formulaic” use of the vocative, which also occurs in other contexts, such as in advertising and marketing, where agents who are serving customers or clients are encouraged to sprinkle their utterances with the addressees’ names (usually preceded by a term of deference such as “Mr.” or “Miss/Mrs./Ms.”).
A particular instance that is worthy of further study is the variable phatic and/or emotive use of the vocative as a feature of an individual speaker’s predilections when addressing an interlocutor. Speakers typically differ from each other in the frequency with which they resort to naming their interlocutors as part of discourse. Constant interspersion of one’s wife’s or husband’s name in addressing a spouse may start as a sign of endearment but may also ultimately devolve into a verbal tic devoid of emotive meaning and destructive of genuine affection. Similarly, the same speech habit in addressing a customer or client can easily lead to annoyance on the part of the addressee and subvert the very psychological affect that the utterer is aiming to engender in order to further their mercantile goal.