This article serves as a basic definition of the corresponding keyword on the site socioeco.org.
Third Sector is a term to designate all those economic activities by the civil society to meet the needs of goods and services provided by neither the public sector (first sector) nor the private sector (second sector). Their economic activities are non-profit (i.e. not aimed at distributing the exceeding to external stockholders).
Note that in Japan the word "third sector" (第3セクター) refers to the joint business set up both by the public and private sector, therefore other expressions, such as "NPO sector" (NPOセクター) are used.