A completely new installation of an operating system or application on a computer. In a clean install of an OS, the hard disk is formatted and completely erased. In a clean install of an application, the older version is uninstalled first. Installing an OS on a new computer or installing an application for the first time is automatically a clean install. Contrast with "in-place upgrade." The Clean Upgrade If the clean install is for a newer version of software already running on the computer, it is also called a "clean upgrade."