The one being addressed. The word that makes a statement into something said to someone.
Old English ēow, the dative and accusative form of the second-person plural pronoun (whose subject form was gē, "ye"), from Proto-Germanic *iwwiz. The old distinction between subject "ye" and object "you" collapsed into a single modern word that now serves singular and plural, subject and object alike.
The word that turns a statement from something said about a person into something said straight to them. Address is what separates a report from a message aimed at someone in particular.