In Habermas's usage, the realm of social life in which private persons come together to form public opinion by rational discussion, independent of the state and the market.
In more detail
In The Structural Transformation of the Public Sphere (1962) Habermas traced a bourgeois public sphere of salons, cafés and journals in eighteenth-century Europe and argued that mass media and commercial pressures later weakened it. In his later democratic theory the informal public sphere supplies opinion that flows into formal institutions. Critics such as Nancy Fraser note its exclusions and argue for plural publics.