In law, it means that a property or benefit has been enjoyed for so long that its owner does not have to prove how they came to own it. The term has been formally defined for some purposes:
In English law and its derivatives, time immemorial means the same as time out of mind, "a time before legal history and beyond legal memory." In 1275, by the first Statute of Westminster, the time of memory was limited to the reign of Richard I (Richard the Lionheart), beginning 6 July 1189, the date of the King's accession. Since that date, proof of unbroken possession or use of any right made it unnecessary to establish the original grant under certain circumstances. In 1832, time immemorial was re-defined as "Time whereof the Memory of Man runneth not to the contrary." The plan of dating legal memory from a fixed time was abandoned; instead, it was held that rights which had been enjoyed for twenty years (or as against the Crown thirty years) should not be impeached merely by proving that they had not been enjoyed before (holding by adverse possession).