Time immemorial is time extending beyond the reach of memory, record, or tradition. The implication is that the thing referred to is, or can be regarded as, indefinitely ancient.

The term has been formally defined for some purposes.

• In English law time immemorial means "a time before legal history, and beyond legal memory" so called; formerly an indefinite time, but in 1276 this time was fixed by statute as the year 1189, the beginning of the reign of King Richard I. Proof of unbroken possession or use of any right since that date made it unnecessary to establish the original grant. In 1832 the plan of dating legal memory from a fixed time was abandoned and the principle substituted that rights which had been enjoyed for full twenty years (or as against the crown thirty years) should not be liable to impeachment merely by proving that they had not been enjoyed before. Source: The public domain Webster's Revised Unabridged Dictionary (1913)
• The Court of Chivalry is said to have defined the period before 1066 as "time immemorial" for the purposes of heraldry.

Compare with: prehistoric

• Cite for the heraldry date
• Another cite for the heraldry usage