This topic encompasses all areas of security that are dedicated to defense, that is$,$ reducing the chances of a system being attacked, as well as reducing the harmful effects of an attack, if it were to happen.

Relevant concepts:

Cryptography $($protection of documents and communications$)$

Access control

Protection against malicious software $($malware$)$

Intrusion Detection Systems

Intrusion Prevention Systems

Firewalls e Web Application Firewalls

Defense against buffer overflows

Defense against social engineering

Backups and System Redundancy

VPNs

Physical Security