Manumission is the act of freeing a slave, done at the will of the owner.
The term is Middle English and is derived from the Latin manumittere. The act of manumission dates back to ancient Rome. Popes, emperors, and minor landholders— all are counted among those who practised it. During the Middle Ages serfs were freed through a form of manumission.
The process differed from time-to-time and from lord-to-lord. High productivity, loyal service, or even buying their way out of service were all reasons for which slaves or serfs received their freedom under manumission.
Manumission was a tenuous process at best. In ancient Rome, freed slaves were not "freeborn" and were still required to grovel in the presence of their former masters. During the Middle Ages serfs, who had obtained their freedom and farmland, would often give up their land in troubled times in exchange for the protection of their former feudal masters. In times of bad harvest, serfs could find themselves, once again, attached to the land of a noble for lack of any other means of survival.
For these reasons, manumission is not the same as emancipation, the freeing of slaves by an act of government (for example, at the end of the American Civil War).