The Oxford English Dictionary, quoted by Fowler, gives-
Quote: such portion of a composition or utterance as extends from one full stop to another.
I altered that by saying from a capital letter to a full stop.
Fowler offers the alternative-
Quote: a combination of words in an analysable grammatical structure.
Fowler offers 10 definitions of which the first 7 are in line with the OED (popular) and the 8th and 9th in line with the grammarians.
The 10th is said to try to-
Quote: reconcile the two by giving a grammarian's definition with a procrustean device for fitting it into apparently unconformable sentences of the 'popular' kind.
It goes on-
Quote:It is more realistic to admit that the two may be irreconcilable; that what may be placed between one full stop and another may lack even an elliptical grammatical structure.
And just to complete your education on this point YL it adds-
Quote:Modern writers show greater freedom than was once customary on what they place in that position. And what of the will to power? /Finally on one small point./ So then./ Now for his other arguments. These, taken from scholarly writings by contemporary men of letters, cannot be denied the right to be called sentences, but it would be straining language to say that they are elliptical in the sense that 'a subject or predicate or verb (or more)' must be 'understood'. Grammarians are free to maintain that no sequence of words can be called a sentence unless it has a grammatical structure, but they should recognize that, except as a term of their art, the word has broken the bounds they have set for it.
So, YL, you and Mac are grammarians and your agreed definition is tautological. As far as modern writing goes you are isolated pedants. I very much doubt that Richard Ellmann, Scott Moncrieff, Terence Kilmartin, Sir Henry Rider Haggard or Anthony Burgess would go into bat for your position and James Joyce certainly wouldn't.
Partridge declines to offer a definition at all.
Grammar is, obviously, necessary for legal prose.