There are many types of relations that were referred to as slavery throughout history. The common, vernacular English definition does refer to chattel slavery and there is an onus during discussion to make oneself clear.

Having said that, equating all slavery to chattel slavery and thus denying that other slavery-like arrangements have existed and continue to exist today is not only a misleadingly US-centric perspective, it also removes the ability to criticize and try to reform social arrangements that severely restrict freedom in subtler and more nuanced ways.

If we're talking about late ancient societies, slavery often meant social subordination to your master and some obligated work.

It is not dissimilar from undischargible debt. In fact, the historians and anthropologists writing about these societies have made the link themselves.