Hebrew שְׁמִטָּה. Literally "a release". Also called Shemitah, Year of release.
Every seventh year the land rests, and debts are let go.
A Sabbath for the ground: every seventh year no sowing, no pruning, and whatever grows of itself is left for the poor and for wild animals. Debts between Israelites are released in the same year. Chronicles reads the seventy years of exile as the land finally taking the sabbaths it had been denied, which is a striking way to account for a national catastrophe.
Every seventh year the fields lie fallow and debts are released, and after seven of those cycles the fiftieth year returns land to the families that sold it. So land was never really sold, only its harvests until the next Jubilee, and Leviticus says the price should follow the harvests left: it drops by a harvest each year, and holds still across a sabbatical year, which has no harvest to sell. Whether the Jubilee was ever actually kept is not recorded.