Time immemorial is a phrase meaning time extending beyond the reach of memory

Memory

In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

, record, or tradition

Tradition

A tradition is a ritual, belief or object passed down within a society, still maintained in the present, with origins in the past. Common examples include holidays or impractical but socially meaningful clothes , but the idea has also been applied to social norms such as greetings...

, indefinitely ancient, "ancient beyond memory or record". The phrase is one of the few cases in the English language where the adjective

Adjective

In grammar, an adjective is a 'describing' word; the main syntactic role of which is to qualify a noun or noun phrase, giving more information about the object signified....

 is a postmodifier

Grammatical modifier

In grammar, a modifier is an optional element in phrase structure or clause structure; the removal of the modifier typically doesn't affect the grammaticality of the sentence....

—some other phrases, such as the legal terms attorney general

Attorney General

In most common law jurisdictions, the attorney general, or attorney-general, is the main legal advisor to the government, and in some jurisdictions he or she may also have executive responsibility for law enforcement or responsibility for public prosecutions.The term is used to refer to any person...

 and court martial, also follow that pattern, largely due to the influence of Norman French.

The term has been formally defined for some purposes.

• In English law
  English law
  English law is the legal system of England and Wales, and is the basis of common law legal systems used in most Commonwealth countries and the United States except Louisiana...
  
   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 I of England
  Richard I was King of England from 6 July 1189 until his death. He also ruled as Duke of Normandy, Duke of Aquitaine, Duke of Gascony, Lord of Cyprus, Count of Anjou, Count of Maine, Count of Nantes, and Overlord of Brittany at various times during the same period...
  
   (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
  The Crown
  The Crown is a corporation sole that in the Commonwealth realms and any provincial or state sub-divisions thereof represents the legal embodiment of governance, whether executive, legislative, or judicial...
  
   thirty years) should not be impeached merely by proving that they had not been enjoyed before (holding by adverse possession
  Adverse possession
  Adverse possession is a process by which premises can change ownership. It is a common law concept concerning the title to real property . By adverse possession, title to another's real property can be acquired without compensation, by holding the property in a manner that conflicts with the true...
  
  ).
• The Court of Chivalry
  Court of Chivalry
  Her Majesty's High Court of Chivalry of England and Wales is a civil court in England. It has had jurisdiction in cases of the misuse of heraldic arms since the fourteenth century....
  
   is said to have defined the period before 1066 as time immemorial for the purposes of heraldry
  Heraldry
  Heraldry is the profession, study, or art of creating, granting, and blazoning arms and ruling on questions of rank or protocol, as exercised by an officer of arms. Heraldry comes from Anglo-Norman herald, from the Germanic compound harja-waldaz, "army commander"...
  
  .