Your definition recalls the discussions in Britain at the end of the 18the century about whether slaves should have their humanity fully recognised in which case the removal of their state of slavery would be required as then Christian ethics could require that to happen.  Of course that is exactly what happened.  The arguments against the 'emancipatory' position was not that they were not human but rather that the abolition of slavery would lead the owners (of property, i.e. slaves) to lose as free labour they suggested would be less productive than slave labour.

I define slavery properly so-called as: the ownership of one person and their descendants by another.  Anything else I would not call slavery.  I do recognise though that other forms of labour such as serfdom are sometimes almost indistinguishable from slavery.  In terms of the actual conditions for example of bonded labourers in the British Empire in the Nineteenth Century there were definitely points at which they may have been materially 'better off' under slavery.

Thank you, Richard, for your answer. But is the ownership for the descendants a necessary condition? Debt slavery in Archaic Greece was a personal matter, not inherited. And the cases of children born from slave mothers and raised up as the master's own children also seem to have place.

one clear demarcation between slavery and serfdom in early modern Spain is that slaves were sold and bought in auctions in public or private spaces and that slaves run their own 'black' confraternities.