Irreligion (meaning not religious) is a term that describes people who reject religion, or who do not have a particular religion.
The term covers all those who answer "none" to questionnaires which ask people about their religion. In the social survey world, they are called "the nones". The term irreligious is an umbrella term, for atheists, agnostics, deists and ignostics.
Irreligion takes many forms, ranging from the casual and unaware to full-fledged philosophies such as atheism, agnosticism, and secular humanism.
Irreligion may be a purely naturalist worldview which denies a belief in anything supernatural. The broadest and loosest definition, is the lack of religious identification (attendance at church, temple etc.). The narrowest and strictest is subscribing to "positive atheism". That might be described as a willingness to state rejection of a deity absolutely. Public statements of belief can be, and are, tailored to the various social climates.
'No religion' groups
There are, at least three different groups of "not religious" people: Agnostics Atheists Deists