Duke University
Duke University: The Color Force
What does color have to do with particle physics? This page explains the concepts of color and the color force, which are fundamental subjects in quantum chromodynamics.
Wolfram Research
Wolfram Science World: Quantum Electrodynamics
In its description of quantum electrodynamics this definition provides the governing equations.
Lawrence Berkeley National Laboratory
Berkeley Lab: Four Fundamental Interactions
Discussion of the four fundamental forces of nature: strong interactions, weak interactions, gravity, and electromagnetism.