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).
A crag is a rocky outcrop. Here are some sentences. The mountain climbers had a tough time on the steep crag. The huge crag loomed over the city. No trees could grow on the crag.
A sentence is a complete clause with a subject and a finite verb. A sentence fragment is not a complete clause, but is part of a clause (often lacking the verb to make a full sentence). For instance,...