Thrall

Norse slaves and indentured servants

A thrall, called þræll in Icelandic, was a person held as a slave or serf. This status contrasts with that of a free man (karl) or a noble (jarl). Thralls were the lowest class within Scandinavian society. A person became a thrall either by enslavement as a prisoner of war, incurring great debt, or being born into the class. As great sources of labor, thralls were prized pieces of property and a great part of the viking era economy. Most families had one or two thralls, though some large households may have had upwards of thirty.
A thrall's conditions and roles varied depending on their master, but thralls were still considered to be human. Saint Hallvard, the patron saint of Oslo, was born the son of a local jarl and martyred while protecting a thrall woman from men that accused her of theft. His actions were regarded positively by greater society, just as the cruelties exerted on the thrall woman regarded highly negatively. Under the law, both a thrall and a karl commanded a wergeld, the monetary penalty for unlawful killing.
A thrall could experience a greater degree of social fluidity than slaves in many other cultures. A thrall could be freed at any time, in their master's will, or purchase their own freedom. Once freed, a thrall was elevated to leysingi, an intermediate class between thrall and karl. While a leysingi would still owe allegiance to his former master's house for at least two generations, the thrall would no longer have to obey all of his former master's wishes, could own property, and could leave property to any children. 
The thrall system was officially abilished in Scandinavia in the mid 1500s CE, largely due to lack of thralls; as the capture of slaves from viking raids and wars ended in the 1200s CE, many thralls continued to purchase their freedom or be granted freedom by their masters or other authorities.

Source:

• Thrall Random House Unabridged Dictionary, 2009
• Junius P Rodriguez, Ph.D. (1997). The Historical Encyclopedia of World Slavery. vol 1. A–K. ABC-CLIO. p. 674.
• Ben Raffield (2019) "The slave markets of the Viking world: comparative perspectives on an ‘invisible archaeology’." Slavery & Abolition, 40:4, 682-705
• Thomas K. Heebøll-Holm (2020) "Piratical slave-raiding – the demise of a Viking practice in high medieval Denmark" Scandinavian Journal of History
• Niels Skyum-Nielsen, "Nordic Slavery in an International Context," Medieval Scandinavia 11 (1978–79) 126-48
• P.H. Sawyer (2002). Kings and Vikings: Scandinavia and Europe AD 700–1100. Routledge. p. 39. ISBN 978-0-203-40782-0.
• St. Hallvard in Catholic Online. (2009)
• Eyrbyggja Saga, Chapter 37.