What term describes the idea, popularized by j.-j. rousseau, that human society originated in an agreement among naturally free individuals to establish the rule of law and civil society?

The correct answer is The Social Contract

It is thoroughly explained in the book the social contract which was one of the main philosophy books that inspired the founding fathers to write the declaration of independence. The constitution was also based around philosophies by Rousseau, as well as people like Locke and Hobbes.