A OS project from MIT. From its home page (see below):
An exokernel eliminates the notion that an operating system should provide abstractions on which applications are built. Instead, it concentrates solely on securely multiplexing the raw hardware: from basic hardware primitives, application-level libraries and servers can directly implement traditional operating system abstractions, specialized for appropriateness and speed.

What follow was the old, inadequate definition of exokernel: someway, during the transition to Cliki, our awareness regarding exokernels was not adequately reported.

A microkernel OS project from MIT, which seems to have the most features divorced from its core of all of them.

An exokernel is not a microkernel. It is a fundamentally different architecture, whereby the kernel is predominently monolithic in nature, but the services normally offered by a monolithic kernel to applications are pushed up into user-space libraries, called library operating systems. This is how an exokernel achieves flexibility and performance gains; applications need only use the parts of the OS that are necessary, rather than being forced to deal with all the abstractions that are normally enforced by monolithic kernels, including microkernels.
-- A N Other

The debate monolithic kernels vs microkernels is uninteresting for Tunes, which is about No-Kernel systems.
-- MaD70

This page is linked from: Microkernel Debate