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Note added at 7 mins (2005-10-13 01:13:57 GMT)
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A variety of language or a level of usage, especially one determined by formality and choice of vocabulary, prononciation, syntax, , according to the social context or standing of the user.

Oxford New Shorter English Dictionary

register -(2) : a portion of such a range similarly produced or of the same quality c : any of the varieties of a language that a speaker uses in a particular social context (register)
diction: 2 : choice of words especially with regard to correctness, clearness, or effectiveness


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Note added at 14 mins (2005-10-13 01:20:21 GMT)
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I think of a register as a kind of sliding scale (it is also used to talk about the musical scale) from lowest/most informal to highest/most formal . . . you would choose your tone, choice of vocabulary, diction, etc, according to the formality (or lack thereof) of the situation.

"While dialects, and therefore accents, vary from speaker to speaker, any given speaker also shows variation within her own speech depending on context. These intra-speaker variations are known as registers.
Halliday identifies three variables that determine choice of register: field (the subject matter), tenor (who the participants are and the nature of the relationships between them) and mode (the channel of communication).
Any or all of the elements of language may vary in different registers: lexicon, syntax, pragmatic rules and phonology, including pitch, volume and intonation. An actor choosing to speak dramatic verse in his or her own native dialect would naturally change register as all of field, tenor and mode differ from "regular" conversation - but there is nothing intrinsic to this that demands a change of dialect or accent." (http://findarticles.com/p/articles/mi_7122/is_200904/ai_n523...

"Linguistic varieties that are linked ... to occupations, professions or topics have been termed registers. The register of law, for example, is different from the register of medicine, which in turn is different from the language of engineering---and so on. Registers are usually characterized solely by vocabulary differences; either by the use of particular words, or by the use of words in a particular sense.
Registers are simply a rather special case of a particular kind of language being produced by the social situation." (Trudgill (1983)

"In linguistics, a Register is where a person talks differently to different people. They may be more polite to people they do not know for example." (http://simple.wikipedia.org/wiki/Register_(linguistics))

"This term has been applied to varieties of language in an almost indiscriminate manner, as if it could be usefully applied to situationally distinctive pieces of language of any kind. […] It is inconsistent, unrealistic, and confusing to obscure these differences by grouping everything under the same heading […]." (David Crystal)

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Some definitions:
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(1) "a variety of a language or a level of usage, specifically, one regarded in terms of degree of formality and choice of vocabulary, pronunciation, and (when written) punctuation, and related to or determined by the social role of the user and appropriate to a particular need or context" (OED)

(2) "a variety of a language that a speaker uses in a particular social context" (Webster's Unabridged)

(3) "A set of specialized vocabulary and preferred (or dispreferred) syntactic and rhetorical devices/structures, used by specific socio-professional groups for special purposes. A register is a property or characteristic of a language, and not of an individual or a class of speakers." (Harold Schiffman, http://ccat.sas.upenn.edu/~haroldfs/messeas/regrep/node2.htm...
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In the Oxford English Dictionary (OED) the entries are classified according to the use of an expression in different language situations. Generally, all entries are classified as 'standard'. Additionally, some expressions are categorized differently according to the particular contexts in which they are appropriately used.

The main register labels in the Oxford Thesaurus of English (2006) are the following:

* Informal: normally used only in contexts such as conversations or letters between friends
* Vulgar slang: informal language that may cause offence […]
* Formal: normally used only in writing such as official documents
* Technical: normally used in technical and specialist language, though not necessarily restricted to any specific field
* Literary: found only or mainly in literature written in an ‘elevated’ style
* Dated: no longer used by the majority of English speakers […]
* Historical: still used today, but only to refer to some practice or article that is no longer part of the modern world
* Humorous: used with the intention of sounding funny or playful
* Archaic: very old-fashioned language, not in ordinary use at all today […]
* Rare: not in common use”

(Oxford Thesaurus of English 2006, Introduction ix)