Hurd 【GNU Hurd】
GNU Hurd is the kernel of GNU operating system. It is the GNU project's replacement for the UNIX kernel. Development on the GNU Hurd began in 1990. The Hurd is a collection of servers that run on top of the Mach microkernel to implement file systems, file access control, network protocols, and other features that are implemented by the UNIX kernel or similar kernels.
HURD is a co-recursive acronym. HURD stand for 'HIRD of Unix-Replacing Daemons'. And then 'HIRD' stands for `Hurd of Interfaces Representing Depth'. The GNU Hurd is free software. It was released under the GNU General Public License.