The Operating System is a base infrastructure software component of a computerized system. It controls all basic operations of the computer (or other electronic devices such as PDA, smartphone, etc.).

A system software component that is external to the operating system kernel. If the phrase is used to refer to the operating system itself, it is redundant because an operating system "is" software.

The first OS installed in a computer. In a virtual machine environment, there may be a host operating system that interacts with the virtual machine monitor, which manages the virtual machines.