Default deny policy: A security strategy where all network traffic or access requests are automatically blocked unless they have been explicitly approved. This approach significantly reduces the risk of unauthorized access or data breaches as it requires explicit permission for any action, but it can also increase administrative overhead due to the necessity of frequent permissions adjustments.

Default Gateway: The device that provides a connection point for devices on a network to communicate with other networks or the Internet. It serves as the routing path for network traffic that's heading outside the local network and is typically a router or other networking device that has the capability to interface with larger networks.

Default Password: A pre-set password provided by the manufacturer of a hardware device or software application that is intended to be used during the initial setup process. Default passwords are often common and well-known, which can pose a significant security risk if not changed after installation or setup. It is considered a critical security practice to change all default passwords to strong, unique passwords to prevent unauthorized access that can result from default password exploitation. Cybersecurity best practices mandate the updating of default passwords as part of the initial configuration of any new system or device.

Defense in Depth: A security strategy that employs multiple layers of defense measures across an organization's technical and procedural boundaries. It is designed to slow down an attack's progress and provide redundant protective measures in case one system fails or is compromised. This includes not just technical controls like firewalls, antivirus software, and intrusion detection systems but also administrative controls like security policies, training, and physical controls like surveillance and secure locks. The concept is based on a military strategy with the same name, where a series of defensive mechanisms are layered to protect valuable assets. Multiple, complimentary, overlapping security measures.

Degaussing: A process by which magnetic fields are used to neutralize or erase data from a magnetic storage medium, such as hard drives or tapes. The process renders the data unreadable, thus ensuring the secure disposal of sensitive information, though it should be noted that once a device has been degaussed, it is typically no longer usable.

Deidentification: A process where personally identifiable information (PII) is removed or anonymized from a dataset. The aim is to protect the privacy of individuals by ensuring that the data cannot be traced back to them. This technique is often used in data analytics and research, where large volumes of data are needed, but the identification of specific individuals is not.