Hardening of the OS is the act of configuring an OS securely, updating it, creating rules and policies to help govern the system in a secure manner, and removing unnecessary applications and services.
Hardening the operating system means making it more secure. This is typically done by removing all non-essential software programs and utilities from the computer, applying the latest patches, deleting unused files, locking down user accounts. While these non-essential programs may offer useful features to the user, if they provide "back-door" access to the system, they must be removed during system hardening.
Although they are important; removing applications, disabling services, patching, hotfixing, and installing service packs are not the only ways to harden an operating system. Administrative privileges should be used sparingly, and policies should be in place to enforce the rules of the organization.
There are hardening checklists available for popular operating systems that administrators can follow. While both Macintosh and Windows operating systems can be hardened, system hardening is more often done on Windows machines, since they are more likely to have their security compromised.