This reference work defines more than 1,200 terms and concepts that have been found useful in past research and theory on the nonprofit sector. The entries reflect the importance of associations, citizen participation, philanthropy, voluntary action, nonprofit management, volunteer administration, leisure, and political activities of nonprofits. They also reflect a concern for the wider range of useful general concepts in theory and research that bear on the nonprofit sector and its manifestations in the United States and elsewhere. This dictionary supplies some of the necessary foundational work on the road toward a general theory of the nonprofit sector.