A unikernel is a specialised, single address space machine image constructed by using library operating systems. More on wikipedia page

Comparison of a traditional OS stack and a MirageOS unikernel

MirageOS is a library operating system that constructs unikernels for secure, high-performance network applications across a variety of cloud computing and mobile platforms.

Code can be developed on a normal OS such as Linux or MacOS X, and then compiled into a fully-standalone, specialised unikernel that runs under a Xen or KVM hypervisor. This lets your services run more efficiently, securely and with finer control than with a full conventional software stack page.

Mirage OS is written in OCaml page.