Declassification: The process by which information that has previously been classified as confidential, secret, or top-secret is downgraded in secrecy level or made entirely public. This process is critical for ensuring that information which no longer requires protection is accessible for public scrutiny or for use in non-sensitive contexts. The procedure is typically controlled by strict protocols to maintain the overall integrity of an organization's information management system.

Decode: This is the process of transforming encoded or scrambled data back into its original format. It's an essential mechanism in communication systems that enables the safe transmission of data. The encoded data can only be interpreted correctly if the receiver has access to the appropriate decoding technique or key.

Decryption: The process of converting encrypted data (ciphertext) back into its original form (plaintext), making it understandable. This is achieved by using a cryptographic key and following the reverse process of the encryption algorithm applied to the data. Decryption is critical for secure communication, allowing only authorized parties with the correct key to access the sensitive information.

Decryption Key: A type of cryptographic key used to reverse the encryption process, converting encrypted data (ciphertext) back into its original form (plaintext). In symmetric-key algorithms, the same key is used for both encryption and decryption, while in asymmetric-key algorithms, a paired private key is used for decryption that is different from the public key used for encryption. Proper handling and storage of decryption keys are crucial for maintaining the confidentiality and security of sensitive data.

Deencapsulation: The process of removing headers or top layers from a packet, frame, or other data transmission unit during its journey through a network's layers in accordance with the layered architecture models such as OSI or TCP/IP. This action is taken by the receiving end of a data transmission to uncover the original data and interpret it correctly.

Default classification: Default classification refers to the security level automatically applied to data or information in the absence of a specific classification label. While it can be the most restrictive, this is not always the case; the default level is determined by an organization's policy and could potentially be open or public if the data is not sensitive. The default classification serves as an initial safeguard until the proper classification can be reviewed and applied based on the data's content and the need for confidentiality.