The buzz-phrase impact investment admits of different definitions. Some people use it in a very vague way: it would be any investment that benefits poor people. On June 16, 2014 Pope Francis, addressing a conference of investors and those who study them, described it more specifically as an attempt to:
… promote the economic and social development of these [disadvantaged] groups through investment funds aimed at satisfying basic needs associated with agriculture, access to water, adequate housing at reasonable prices, as well as with primary health care and educational services. Investments of this sort are meant to have positive social repercussions on local communities, such as the creation of jobs, access to energy, training and increased agricultural productivity. The financial return for investors tends to be more moderate than in other types of investment.
A good description. Francis went on to praise investors who operate this way. He also excoriated speculators who don’t. You can read it all here.