An ergastulum was a Roman

Roman Empire

The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

 building used to hold in chains dangerous slave

Slavery

Slavery is a system under which people are treated as property to be bought and sold, and are forced to work. Slaves can be held against their will from the time of their capture, purchase or birth, and deprived of the right to leave, to refuse to work, or to demand compensation...

s, or to punish other slaves. The ergastulum was usually subsurface, built as a deep, roofed pit - large enough to allow the slaves to work within it and containing narrow spaces in which they slept. They were common structures on all slave-using farms (latifundium). The etymology is disputed between two possible Greek roots 'ergasterios' (workshop) and 'ergastylos' (pillar to which slaves were tethered).

The ergastulum was made illegal during the reign of Hadrian

Hadrian

Hadrian , was Roman Emperor from 117 to 138. He is best known for building Hadrian's Wall, which marked the northern limit of Roman Britain. In Rome, he re-built the Pantheon and constructed the Temple of Venus and Roma. In addition to being emperor, Hadrian was a humanist and was philhellene in...

.

The term is also used to describe any small Roman prison.