Complex sentence: The American Humanist Association (AHA) is an educational organization in the United States that advances Humanism as defined by the International Humanist and Ethical Union (IHEU), a multinational coalition of which it is a founding member.
Split into simpler sentences: The American Humanist Association (AHA) is an educational organization in the United States that advances Humanism. AHA advocates Humanism as defined by the International Humanist and Ethical Union (IHEU), a multinational coalition of which it is a founding member.