dual boot
also: multi-boot, dual-boot system
A computer configuration where two different operating systems are installed on separate partitions of the same disk, allowing the user to choose which one to boot at startup.
Dual boot enables a single machine to run multiple operating systems by partitioning the hard drive and installing each OS independently. At boot time, a bootloader (like GRUB) displays a menu letting you select which OS to start. Each OS runs in its own partition and remains unaware of the other.
A common example is installing both Linux and Windows on the same computer. You might allocate 200GB to Windows on one partition and 300GB to Linux on another, with a small shared EFI partition for the bootloader. Upon powering on, you choose which system to load.
Important considerations: you can only run one OS at a time (for true simultaneous multi-OS use, consider virtualization), disk space must be carefully allocated during installation, and data sharing between systems requires a common filesystem or external storage.