Nowadays, society faces complex social, economic, and environmental problems which the traditional sectors of activity (public, private and third sector) are not able to solve alone. To respond to these challenges, their missions and strategies have been converging, leading to a phenomenon known as the blurring of boundaries between sectors. Together, they give rise to a new and promising sector in society: the fourth sector. The literature on this new sector is still scarce, even though its impact is already being felt all over the world. The concept of the fourth sector is in itself broad and clearly lacking defined borders. In addition, there are three currents in the literature that use this term with different meanings.This paper aims to contribute to the theoretical knowledge on the fourth sector by clarifying its definition and framing its activity. A qualitative methodology was carried out by using the papers featuring the topic “fourth sector”, indexed to the Web of Science, to generate a textual corpus that led to a similarity analysis. Subsequently, written interviews were conducted with seven academic specialists on the fourth sector topic, selected through the snowball sampling method, to complete the data obtained through the literature review. These data were added to the initial textual corpus, and a new similarity analysis was generated. It is concluded that the fourth sector as a spectrum of hybrid organizations is currently the most accepted approach, and the existence of an activity that combines a business framework with a social purpose seems to be its main characteristic. So, the main implication is related with significant opportunities that have yet to be fully explored, considering the extensive rise of the fourth sector.