"Entail" or "entailment," more strictly known as "fee tail" in legal discourse, is an obsolescent practice in English common law, by which an estate of real property cannot be sold, willed outside the family, or otherwise alienated by its owner, and must pass to the owner's (usually male) descendents upon his death. Entail is perhaps best known as a serious financial problem faced by the families of Jane Austen's female protagonists.
The term "fee tail" comes from the Medieval Latin feodum talliatum, which literally means "property which has been cut out." The difference between "fee tail" and "fee simple" (the inheritance practice most commonly used today) is that in "fee tail" the property is granted "to A and the heirs of his body," whereas in fee simple, the phrasing is "to A and his heirs." The crucial difference is that in fee tail the heirs must be biological children begotten by the owner, whereas in fee simple the heirs can be anyone designated as such by the owner. Land granted under fee tail was said to be "in tail" and was deemed "entailed" to the biological offspring of the owner.
How it worked
The whole point of entail was so that the landed nobility of feudal times could make sure that land would remain in their family forever. Otherwise, all it would have taken was one foolish young fellow whose father died young to sell of his land to make some quick money, and the family's primary sources of wealth and power would be entirely dissipated.
In practice, entail had many powerful protections for the family who owned the land, because any rights to the land granted by the current owner immediately were lost upon that owners death. For example, it would be foolish for anyone to grant a mortgage on entailed land because as soon as the current owner died, the children who inherited the land would have no obligation to repay the mortgage since their interest was considered prior in right over the mortgage. Similarly, any agreement struck between the owner and a tennant was only good for the duration of the owners life, after which point the new owner would have no obligations to uphold the agreement.
Although in the simplest form of entail, the land was entailed to any offspring of the owner, the most typical form of entail used was "fee tail male," in which only male offspring could inherit the land, although it was possible to have "fee tail female," in which only daughters could inherit, as well as "fee tail special" in which other conditions were attached, such as only allowing the land to pass to legitimate children born in wedlock.
Entail created problems for families when the owner of the land failed to have children which survived him, or more typically, failed to have a son. In this case, the land would go back up the family tree through previous owners until their sons or the sons of their sons were found. This was the problem faced by the Bennets in Jane Austen's Pride and Prejudice - since Mr. Bennet had only daughters, all his land was entailed to a cousin, Mr. Collins, and thus his daughters were threatened with the prospect of utter impoverishment upon their father's death.
A brief history of entail
Although forms of entail existed in England prior to the introduction of Norman Law, the law of entail which held sway in Britain for much of recent history was created by the Statute of Wesmtinster II in 1285. The chapter of the statute in which the law appeared was called De Donis Conditionalibus (Concerning Conditional Gifts), because entailed lands were gifts granted in perpetuity to a certain family line only on the condition that they would continue to have biological heirs.
The "Law of Entail" as laid out in 1285 was a feudal statute best suited to the needs of feudal times. It was never popular with the monarchy, the merchant classes which arose in later years, or often with the entailed landholders themselves, who might wish to sell their land. Already, by the 14th century, crafty lawyers had developed an elaborate legal recourse called "Common Recovery," which used collaborative lawsuits and legal fictions to take land out of entail and convert it to fee simple.
The practice of fee tail fell out of use quite early on in urban areas, where it was replaced by fee simple, but entail held on for centuries among the landed gentry in rural areas, until it was finally abolished in England by statute in 1925. Interestingly, since all the American states except for Louisiana inherited English common law, most American states at one time allowed entailment. However, since entail was seen as an undemocratic practice of the English aristocracy, most states abolished it quite early on, well before England did (New York, for example, abolished entail in 1782). Nevertheless, today four US states still allow entailment of land - Delaware, Massachusetts, Maine, and Rhode Island - although obviously, it is hardly ever practiced. In most other states, any attempt to create a fee tail automatically results in a fee simple.