Stephanus pagination is the system of reference and organization used in modern editions and translations of Plato

Plato

Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

 (and less famously, Plutarch

Plutarch

Plutarch then named, on his becoming a Roman citizen, Lucius Mestrius Plutarchus , c. 46 – 120 AD, was a Greek historian, biographer, essayist, and Middle Platonist known primarily for his Parallel Lives and Moralia...

). Plato's (and Plutarch's) works are divided into numbers, and each number will be divided into equal sections a, b, c, d and e. As such, this system is often used to reference Plato - for example, Symposium 172a would refer the reader to the opening of Plato's Symposium

Symposium (Plato)

The Symposium is a philosophical text by Plato dated c. 385–380 BCE. It concerns itself at one level with the genesis, purpose and nature of love....

.

This system of pagination is based on an edition of Plato by Henricus Stephanus (Henri Estienne).