A KDL is a term used on BeOS

BeOS

BeOS is an operating system for personal computers which began development by Be Inc. in 1991. It was first written to run on BeBox hardware. BeOS was optimized for digital media work and was written to take advantage of modern hardware facilities such as symmetric multiprocessing by utilizing...

/Haiku

Haiku (operating system)

Haiku is a free and open source operating system compatible with BeOS. Its development began in 2001, and the operating system became self-hosting in 2008, with the first alpha release in September 2009, the second in May 2010 and the third in June 2011....

 and possibly other operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

s for a serious kernel error which causes the computer to drop into the kernel debugger

Kernel debugger

A kernel debugger is a debugger present in some kernels to ease debugging and kernel development by the kernel developers. A kernel debugger might be a stub implementing low-level operations, with a full-blown debugger such as gdb, running on another machine, sending commands to the stub over a...

. KDL stands for Kernel Debug(ger) Land.

While a KDL brought about by a buggy driver which is not crucial, such as a filesystem driver, can be escaped from (with the command 'es'), it usually leaves the user needing to reboot.