From its README:

Coin is a 3D graphics library with a C++ Application Programming
Interface based on the Open Inventor 2.1 API.  Open Inventor, for those
who are not familiar with it, is an OpenGL-based, retain-mode, 3D model
scene-graph rendering and interaction library, which has become the de
facto standard graphics library for 3D visualization and visual
simulation software in the scientific and engineering community.

WWW: http://www.coin3d.org/