The most honest answer to this question: it depends upon who you ask. Most reliable definitions will surely get around to using the words “subject” and “predicate,” or will talk about a complete thought, or will say something about clauses and relationships. Or, if you happen to be Stanley Fish, and you are writing a book about how to write and read sentences, you might define the sentence as 1) “an organization of items in the world” and 2) “a structure of logical relationships” (How to Write 16). Ok, you say.
The other way to define the sentence would be to turn to a poem, in this case the poem “Permanently” by Kenneth Koch that tells the story of how relationships come together to create what we call the sentence:
One day the Nouns were clustered in the street.
An Adjective walked by, with her dark beauty
The Nouns were struck, moved, changed.
The next day a Verb drove up, and created the Sentence.
Some times stories can help. Technical knowledge (or definitions) in the abstract, as Fish will go on to say in his chapter “Why You Won’t Find the Answer in Strunk and White,” often “yields only the illusion of understanding” (19). This is why when one has a definition in hand the next move will be classification. Here is a very partial list drawn up by Fish:
There are short sentences and long sentences, formal sentences and colloquial sentences, sentences that satisfy expectations and sentences that don’t, sentences that go in a straight line and sentences that surprise, right-branching sentences and left-branching sentences, sentences that reassure and sentences that disturb, quiet sentences and sentences that explode like hand grenades, sentences that invite you in and sentences that exclude you, sentences that caress you and sentences that assault you, sentences that hide their art and sentences that ask readers to stand up and applaud. (44)
What is it that you are trying to do, anyway? For once you figure out what you are trying to do then you will be ready to find the form that will be the answer to the question. It is up to you, as Edgar Allen Poe remarks in his 1846 essay “The Philosophy of Composition” (1846):
Of the innumerable effects or impressions of which the heart, the intellect or (more generally) the soul is susceptible, what one shall I, on the present occasion, select?