A KDL is a term used on BeOS/Haiku and possibly other operating systems for a serious kernel error which causes the computer to drop into the kernel debugger. 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.
References
KDL Wikipedia(Text) CC BY-SA