Generally, there were three kinds of labor considered slavery by modern standards in the United States.
There was outright forced labor which is what happened to the Africans and Native Americans. There was indentured servitude in which poor families up until the early 19th century paid for their travel to America via unpaid labor that was often at interest, unfair, and lasted generations. Then there was unpaid labor or the scrip system which mostly took place in company mine and factory towns in which people were paid company credit at interest to create a form of wage slavery. That lasted till 1948.