noun

1. (narrowly) Belief that no deities exist (sometimes including rejection of other religious beliefs).

2. (broadly) Rejection of belief that any deities exist (with or without a belief that no deities exist).

3. (very broadly) Absence of belief that any deities exist (including absence of the concept of deities).

4. Absence of belief in a particular deity, pantheon, or religious doctrine (notwithstanding belief in other deities).