The GNU project's replacement for the Unix kernel. The Hurd is a collection of servers that run on the Mach microkernel to implement file systems, network protocols, file access control, and other features that are implemented by the Unix kernel or similar kernels such as Linux. The GNU C Library provides the Unix system call interface, and calls the Hurd for services it can't provide itself.
The Hurd aims to establish a framework for shared development and maintenance, allowing a broad range of users to share projects without knowing much about the internal workings of the system - projects that might never have been attempted without freely available source, a well-designed interface, and a multi-server-based design.
According to Thomas Bushnell, BSG, the primary architect of the Hurd: 'Hurd' stands for 'Hird of Unix-Replacing Daemons' and 'Hird' stands for 'Hurd of Interfaces Representing Depth'. Possibly the first software to be named by a pair of mutually recursive acronyms.
[June 1994 GNU's Bulletin].