An atheist/antitheist is a person who denies or disbelieves the existence of a supreme being or beings.
Agnosticism is the doctrine that certainty about first principles or absolute truth is unattainable and that only perceptual phenomena are objects of exact knowledge. It is the belief that there can be no proof either that God exists or that God does not exist.
Atheistic thought found recognition in a wide variety of other broader philosophies such as secular humanism, nihilism, naturalism, anarchism, objectivism, logical positivism, feminism, Marxism, scientism, structuralism etc. (For further investigation, detailed summaries of each are available on the internet.)