A mastermind group is also sometimes called a Personal Growth study. It is a group of people that are meeting either in-person or virtually who are focused on taking results in their lives to a new level. Having the support and ideas from other focused and driven people allows us to see thing differently and to get a new perspective on goals and action plans.
These growth studies are based on multiple topics, but always designed to stretch and grow everyone committed to the group. For most of them I use one of John C. Maxwell's books and use that as the base of the personal growth study.