Just as peeling an onion requires getting through several skins, an attacker must breach each independent layer to reach the "crown jewels": Defense-in-Depth: A Necessary Approach to Cloud Security
In computer security, the "onion" analogy represents a strategy, where multiple redundant security layers protect a central core of critical data. The 7 Layers of the Security Onion The Multi-Layered Onion of Computer Security – AZMATH