System hardening is a step by step
process of securely configuring a system to protect it
against unauthorized access, while taking steps to make
the system more reliable. Generally anything that is
done in the name of system hardening ensures that the
system is both secure and reliable.
System hardening is necessary since "out of the box",
some operating systems tend to be designed and installed
primarily to be easy to use rather than secure. Most but
not all systems can have security measures enabled that
will make them suitable for high security and high
reliability environments.