Hebrew יוֹבֵל. Literally "a ram's horn". Also called Yovel, Year of Jubilee.
Every fiftieth year: land returns to the family that lost it, and everyone sold into service goes free.
After seven sabbatical cycles the fiftieth year is announced on the Day of Atonement with a horn, and two things happen that no other law in the ancient world attempts: land sold reverts to the family it belonged to, and Israelites in bonded service go home. The effect is that land could not really be sold, only leased until the next Jubilee — which is why Leviticus says the price must be reckoned by the number of harvests left. Whether it was ever actually kept is not recorded.
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.
Disputed. The law is detailed, but no narrative anywhere in the Old Testament describes a Jubilee being observed, and scholars disagree about whether it functioned as law or as ideal.