De Interpretatione Ch. 4 - "What is a Sentence?" - Aristotle

Definition of a Sentence

A sentence is significant speech, of which this or that part may have meaning—as something, that is, that is uttered but not as expressing a judgement of a positive or negative character. 

Aristotle, again, begins with a simple definition; this time for a sentence. A sentence (1) is a meaningful utterance (2) in which its parts have meaning independently of the whole if they are broken down. 

Let me explain this more fully. Take ' mortal.' This, doubtless, has meaning but neither affirms nor denies; some addition or other is needed before it can affirm or deny. But the syllables of 'mortal' are meaningless. So it is also with 'mouse,' of which '-ouse ' has no meaning whatever and is but a meaningless sound. But we saw that in composite nouns the particular parts have a meaning, although not apart from the whole. 

This has already been explain in the previous chapters. 

Sentence Mood

But while every sentence has meaning, though not as an instrument of nature but, as we observed, by convention, not all can be called propositions. We call propositions those only that have truth or falsity in them. A prayer is, for instance, a sentence but neither has truth nor has falsity. Let us pass over all such, as their study more properly belongs to the province of rhetoric or poetry. We have in our present inquiry propositions alone for our theme.

There are different "moods" for sentences in language. Only the indicative mood deals with truth and falsity claims about the world. Commands or wishes or questions do not really make truth claims.