The third place is a social space separate from the two usual social environments of home ('first place') and the workplace ('second place'). The concept is generally associated with Ray Oldenburg and his book The Great Good Place. Oldenburg argues that third places are important for civil society, democracy, civic engagement, and establishing feelings of a sense of place. Read the wikipedia entry for Third Place.

Third Place and Third Space are often used interchangeably, but Third Space  Theory is also a postcolonial sociolinguistic theory of identity and community attributed to Homi K. Bhabha. Read the wikipedia entry for Third Space Theory.