A debtors' prison is a prison

Prison

A prison is a place in which people are physically confined and, usually, deprived of a range of personal freedoms. Imprisonment or incarceration is a legal penalty that may be imposed by the state for the commission of a crime...

 for those who are unable to pay a debt

Debt

A debt is an obligation owed by one party to a second party, the creditor; usually this refers to assets granted by the creditor to the debtor, but the term can also be used metaphorically to cover moral obligations and other interactions not based on economic value.A debt is created when a...

.

Prior to the mid 19th century debtors' prisons were a common way to deal with unpaid debt.
Debt bondage

Debt bondage

Debt bondage is when a person pledges him or herself against a loan. In debt bondage, the services required to repay the debt may be undefined, and the services' duration may be undefined...

 was common in ancient Mediterranean societies, including Greek city-states. Economic crises in Athens

Athens

Athens , is the capital and largest city of Greece. Athens dominates the Attica region and is one of the world's oldest cities, as its recorded history spans around 3,400 years. Classical Athens was a powerful city-state...

 around 600 BC had resulted in so many Athenians sold into slavery that Solon

Solon

Solon was an Athenian statesman, lawmaker, and poet. He is remembered particularly for his efforts to legislate against political, economic and moral decline in archaic Athens...

 enacted the seisachtheia

Seisachtheia

Seisachtheia was a set of laws instituted by the Athenian lawmaker Solon in order to rectify the widespread serfdom and slaves that had run rampant in Athens by the 6th century BC, by debit relief...

 which banned the practice of debt bondage and returned Athenians to their land as free men, at least according to Solon's own claims.

Under the early Roman Republic

Roman Republic

The Roman Republic was the period of the ancient Roman civilization where the government operated as a republic. It began with the overthrow of the Roman monarchy, traditionally dated around 508 BC, and its replacement by a government headed by two consuls, elected annually by the citizens and...

, a person could pledge himself as a collateral for a loan, in a type of contract called Nexum

Nexum

Nexum was a debt bondage contract in the early Roman Republic. The debtor pledged his person as collateral should he default on his loan. Nexum was abolished by the Lex Poetelia Papiria in 326 BC.- The contract :...

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