Hardening refers to providing various means of protection in a computer system.Protection is provided in various layers and is often referred to as defence in depth. Protecting in layers means to protect at the host level,the application level, the operating system level, the user level, the physical level and all the sublevels in between. Each level requires a unique method of security.A hardened computer system is a more secure computer system.
Most popular hardening are as following,
Firewall Security Hardening,
VMware Security Hardening,
Apache Web Server Hardening,
System Hardening and Hardening Clients Operating Systems.