The nobi (奴婢) were the lowest class of Joseon Korea, the word combining "no" (奴), a male slave, and "bi" (婢), a female slave. Under the yangcheonje system that split society into free-born (yangin) and base people (cheonin), they were the representative base people, and were treated all but as property — objects of sale, inheritance, and gift.
Public and private, resident and non-resident
Nobi were divided by ownership into public nobi (gongnobi), belonging to state institutions, and private nobi (sanobi), belonging to individuals and yangban houses. By living arrangement they were also split into resident nobi, who lived in the master's household doing housework and farming, and non-resident nobi, who kept their own household and instead paid a tribute called singong. Non-resident nobi could accumulate their own property and live fairly independently — quite unlike the image of a "servant" one might imagine.
How status was passed down
Because nobi status was hereditary, the rules that set it effectively determined the slave population. For a long time the principle of ilcheonjeukcheon — one base parent makes the child base — was in force, while the nobi jongmobeop, which had the child follow the mother's status, was enacted and repealed at different times. Under such rules of inheritance, nobi status was hard to escape.
Far more numerous than one might think
Nobi made up a substantial part of the population from early to late Joseon. Scholarly estimates put them at as much as 30–40% of the total depending on the period and region, and some studies of local household registers show the share running very high at times. In other words, a large portion of Joseon's people were nobi — and many Koreans today likely descend from them.
The abolition of slavery
The nobi system was dismantled not at once but step by step. In 1801, under King Sunjo, some 60,000 public nobi of the central offices were freed; in 1886 the inheritance of nobi status was banned; and finally, in the 1894 Gabo Reform, the status system and slavery were abolished in full. Having propped up the very bottom of the hierarchy for centuries, the nobi are historically significant as the labor force that in reality sustained Joseon society.