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).
The most common context is in the setting of a civil lawsuit. A majority of the evidence must point to the defendant being the cause of whatever loss the plaintiff is claiming. Thus: "The defendant...
A sentence is a sentence because it has a subject (noun or noun phrase), a predicate ( verb) and punctuation as well as the other parts of speech like pronouns ( she, it him, her, they, them)...