The bootloader officially called GNU’s GRand Unified Bootloader is GNU GRUB.
Usually shortened to GRUB, GNU GRUB is a bootloader package from the GNU Project. It runs before the operating system and can present a menu of installed systems or kernel configurations. After the user selects an entry, GRUB loads the chosen kernel and usually an initramfs, then transfers control to the operating system.
The project grew from an earlier package called the Grand Unified Bootloader, a name that played on “grand unified theory.” It was initially developed by Erich Boleyn for work involving GNU/Hurd. In 1999, GRUB became an official GNU Project package, and its development opened more broadly to the public.
GRUB is especially associated with Linux dual-boot and multi-boot installations, but it supports many platforms and operating systems. GRUB Legacy and GRUB 2 are distinct generations; GRUB 2 is a substantial rewrite with modular code, scripting, broader platform support, and support for both traditional BIOS and UEFI boot paths. systemd-boot, Syslinux, and LILO are alternative bootloaders, not expansions of this name.