From the Website at http://sam.zoy.org/elk/

What is Elk?

Elk is an implementation of the Scheme programming language. In contrast to existing, stand-alone Scheme systems Elk has been designed specifically as an embeddable, reusable extension language subsystem for applications written in C or C++. Elk is free software, and can be used, modified and distributed in almost any way. See the licence file for the exact conditions.

Developers using Elk can deliver applications with different components written in different languages, such as an efficient core written in C or C++ and an extensible user interface layer implemented in Scheme. To help building hybrid application architectures, Elk supports a tightly-knit interworking of the C/C++ parts of applications with Scheme code.

Elk is also useful as a stand-alone Scheme implementation, in particular as a platform for rapid prototyping of X11-based Scheme programs.