Unikernel is a software architecture that compiles an application and the minimal operating system functions it needs into a single, specialized machine image that runs directly on a hypervisor or bare metal. Unlike traditional operating systems that provide general-purpose services to many applications, a unikernel is built for one application: it includes only the drivers, network stack, and system libraries that application actually uses, discarding everything else. The result is an image that is smaller, faster, and more secure than a conventional VM — but also less flexible, since it cannot run arbitrary programs.
Unikernels represent the logical extreme of a trend toward application-specific system images. Where a traditional VM runs a full operating system shared by many applications, and a container shares the host's kernel while isolating user-space, a unikernel eliminates the host kernel entirely and compiles its necessary functions into the application itself. This is not merely an optimization; it is a reconceptualization of the boundary between program and environment. The operating system ceases to be a shared platform and becomes a private library.
See also: Operating system, Kernel, Microkernel, Library OS, MirageOS