An immutable ledger is a type of database that cannot be changed once it has been written. This means that every transaction or record is permanent and cannot be altered or deleted, providing a reliable and trustworthy source of truth. This technology is widely used in cryptocurrency systems and various applications where data integrity and security are paramount. In practice, upon each transaction, a new record is created, and previous records remain unchanged, thereby creating a transparent and complete history of all transactions. The concept of immutability is achieved through cryptographic techniques, leveraging hashing functions and consensus mechanisms to ensure that all network participants agree on the contents of the ledger.