Here’s a good definition, in six parts:
1. No single religious community enjoys primacy or a monopoly with respect to government endorsement, support, or privilege.
2. Religious actors [i.e. people acting according to religious tenets and beliefs] within a state (some or all) enjoy the freedom to carry out their most distinctive activities–worship and other rituals, education, and public expression, the building of places to worship, missionary work, artistic expression, cultural expression and distinctive dress, and the conduct of civil society activities, including running hospitals, orphanages, services for the poor, and care for elders.
3. Religious actors enjoy the autonomy to create their offices and appoint their leadership.
4. Religious actors lack any standing prerogatives over the appoint of state officials or the making of public policy.
5. Religious actors enjoy autonomy in raising, governing, and spending finances.
6. Religious actors enjoy a transnational structure that strengthens their power vis-a-vis the state.
These six points are taken from Monica Duffy Toft, Daniel Philpott, Timothy Samuel Shah, God’s Century: Resurgent Religion and Global Politics. I’m only partway through the book, but it seems well-researched, persuasive, and increasingly relevant.