A more thorough explanation:

Definition: Beyond memory or record; very old.

Examples:

• Some traditions have been passed down from time immemorial.
• The ruins of the ancient city date back to immemorial times.

The term "immemorial" refers to something that is so old that it is beyond memory or record. The examples illustrate this by showing traditions and ruins that have been around for a very long time, possibly even before written records were kept. The term can also refer to a point in time beyond which legal memory cannot go, or simply to a very long time.