The term “atheist” was originally used by the Church of Scientology to describe Scientologists who were not Scientologists, according to Wikipedia.
Now, “agnostic” is also a catch-all term that is used to describe atheists or agnostics who are not religious.
The word “atheism” is not used in the official dictionary definitions for atheism, agnosticism, or agapostasy.
But it does appear in several dictionaries and other publications.
The official definition of atheism by the Oxford English Dictionary defines the term as follows: Atheism is a non-theistic belief system based on a rejection of supernaturalism.
It is not necessarily a religious belief system.
It may include a number of non-religious beliefs that are not theistic.
Theistic religion and secularism are distinct.