What is Nullum Tempus Occurrit Regi?

Legal Definition
Nullum tempus occurrit regi ("no time runs against the king"), sometimes abbreviated nullum tempus, is a common law doctrine originally expressed by Bracton in his De legibus et consuetudinibus Angliae in the 1250s. It states that the crown is not subject to statutes of limitations. This means that the crown can proceed with actions that would be barred if brought by an individual due to the passage of time. The doctrine is still in force in common law systems today. In republics, it is often referred to as "nullum tempus occurrit reipublicae".
-- Wikipedia
Legal Definition
No time bars the king. See 160 111 77, 52 Am. St. Rep. 306, 43 N. E. 382.
-- Ballentine's Law Dictionary