Microkernel - Nanokernel

The term nanokernel or picokernel historically referred to:

1. A kernel where the total amount of kernel code, i.e. code executing in the privileged mode of the hardware, is very small. The term picokernel was sometimes used to further emphasize small size. The term nanokernel was coined by Jonathan S. Shapiro in the paper The KeyKOS NanoKernel Architecture. It was a sardonic response to Mach, which claimed to be a microkernel while being monolithic, essentially unstructured, and slower than the systems it sought to replace. Subsequent reuse of and response to the term, including the picokernel coinage, suggest that the point was largely missed. Both nanokernel and picokernel have subsequently come to have the same meaning expressed by the term microkernel.
2. A virtualization layer underneath an operating system; this is more correctly referred to as a hypervisor.
3. A hardware abstraction layer that forms the lowest-level part of a kernel, sometimes used to provide real-time functionality to normal OS's, like Adeos.

There is also at least one case where the term nanokernel is used to refer not to a small kernel, but one that supports a nanosecond clock resolution.