Hardening is the process of securing the operating system and applications through the installation of patches and updates, plus making changes to the settings to ensure that they are secure. Virtual images are “templates” that virtual machines are created from.
Hardened virtual images means that all virtual images derived from them are “secure by default”, or at least more secure than stock images. The hardened images should meet the organization’s security standards and be patched to a reasonable level. They want to know that all virtual images start from a secure state and hopefully remain secure throughout the system’s lifecycle.
It is expected that the virtual images are hardened to standards set by either industry requirements/regulations or to company standards, only if they are more stringent. It is also expected that the images are up-to-date on all patches when created, and that the images are updated with new patches on a regular basis.