Defense In-Depth

Locating mutually supportive defense positions in such a manner as to absorb and progressively weaken an attack, prevent initial observations of the entire position by the enemy, and allow the commander to maneuver his reserve.

The use of security technologies at multiple points in the network to reduce the overall probability that attacks can penetrate business-critical resources. In e-mail environments, defense in depth helps administrators ensure that malicious code or activities can be stopped at multiple "check points" in the e-mail infrastructure, decreasing the likelihood that a single threat can slip through. [Exchange 2007]