The term slave labor may be used to define either work or workers.
In the former, slave labor is any unreasonably demanding work (usually in poor working conditions) with little to no pay. Child and prison labor alike are generally considered slave labor.
The board of corrections is looking into allegations that prisons force inmates to do slave labor.
This term might also be used more informally
to describe a job one hate
They work me like a dog over in accounting. I swear, this job is slave labor!
In the latter, slave labor describes a workforce of enslaved people who are made to work against their will.
Some tobacco farmers refused to give up their slave labor after the war.