Defense in depth is the act of using multiple security measures to protect the integrity of information. This method addresses vulnerabilities in technology, personnel and operations for the duration of a systems life cycle.
Techopedia explains Defense In Depth
This is simply the use of multiple layers of defense placed throughout an information system. With multiple layers the outermost layers are the first line of defense and work well against the smallest and most common attacks. More powerful and unconventional attacks get past the first few layers but will be stopped by the deeper, more concentrated layers. The concept was conceived by the National Security Agency (NSA) as a comprehensive approach towards information and electronic safety.