A discursive sentence is one that is rambling or verbose, as opposed to concise and clear.
Adam started to behave very discursive when a cute girl walked by. He was even tripping over his own feet.
The political candidate was discursive when asked what his position was on poverty.
When it has a subject, verb, punctuation, capitalization, a predicate, and a complete thought. A sentence is a sentence when it is handed down by a judge (or a magistrate).