A partition on which you can install a bootable operating system and its associated file system. Primary partitions cannot be subdivided into further segments, as extended partitions can.
The partition table on a drive’s master boot record can contain entries for up to four primary partitions or three primary and one extended partition. Only one primary partition at a time can be the active partition (contain the currently running operating system files).
In Microsoft Windows NT, the active partition is also known as the system partition. Primary partitions are used in multiboot systems to isolate the files of each operating system from one another.