Complex sentence: There is no single written expression or definition from this era that exactly corresponds with a modern understanding of the term "republic" but most of the essential features of the modern definition are present in the works of Plato, Aristotle, and Polybius.
Split into simpler sentences: There is no single written expression or definition from this era that exactly corresponds with a modern understanding of the term "republic". However, most of the essential features of the modern definition are present in the works of Plato, Aristotle, and Polybius.