Defence in depth is implementing several layers of defence, including both administrative aspects (procedures, instructions, sanctions, access control rules, confidentiality rules) and technical aspects (multiple layers of protection together with measures for detection and delay) that adversaries would have to overcome or circumvent to achieve their objectives. 
Defence in depth is the combination of multiple layers of systems and measures that have to be overcome or circumvented before nuclear security is compromised.
Defense In-Depth is the approach of using multiple layers of security to guard against failure of a single security component. 
Defence in Depth: The strategy of forming layers of protection for an asset. 
Defense-in-Depth is an information security strategy integrating people, technology, and operations capabilities to establish variable barriers across multiple layers and missions of the organization. 
Defense-in-depth is the provision of multiple security protocols, especially in layers, with the intent to delay if not prevent an attack. 
Defense-in-depth: A principle for building systems stating that multiple defensive mechanisms at different layers of a system are usually more secure than a single layer of defense. 
- ↑ IAEA - Nuclear Security Series Glossary Version 1.3 (November 2015)
- ↑ Oman CERT Glossary
- ↑ Abu Dhabi Safety and Security Planning Manual
- ↑ NIST Special Publication 800-53 Rev 4: Security and Privacy Controls for Federal Information Systems and Organizations (April 2013)
- ↑ ISA99 Committee Master Glossary.
- ↑ OWASP Glossary