In terminology, atheist is a person who does not believe in the existence of god or gods; and does not believe in the existence of one God viewed as the creative source of man and the world who transcends yet is immanent in the world. 
See main: Atheism types(add discussion)
See main: Godless belief systemsThe typical "atheist" tends to fall in the 5 to 8 Dawkins scale number range. Once a person admits to being an atheist or thereabouts, the question that immediately comes to the fore, is what that person does believe in, i.e. what that person's non-deity based belief system is — being that “atheism”, by definition, is not a belief system, though some would like to conceive that it is. Some, such as as atheist Thomas Jefferson, who characterized himself in letter as an "Epicurean materialist" (1820), historically, have opened up publicly about their godless belief system. The following is a list of known belief systems or philosophies adhered to by atheists:
Other terms include: physicalism and (add) which need placement above, somewhere.
Belief system | Godless Date Description Example Believer[s] Basis Spiritualism 2700BC "spirit" based
Imhotep Heraclitean (Ѻ) 475BC "flux and fire"
Heraclitus Materialism 460BC Leucippus Empedoclean 420BC 4 elements + 2 forces
Empedocles (Ѻ) Standard model Epicureanism 270BC "pleasure principle"
Epicurus, Amafinius, Horace—whose famous statement Carpe Diem ("Seize the Day"), supposedly, illustrates the philosophy—Lucretius, Virgil, Julius Caesar, Pierre Gassendi, Denis Diderot, Jeremy Bentham, Christopher Hitchens, and Michel Onfray. (Ѻ) Atomic theory Cartesian atheist 1660 Franciscus van den Enden (Ѻ) Spinozan 1676 "god or [=] nature" belief system Benedict Spinoza, Albert Einstein Newtonian atheist 1686 Pierre Laplace Epicurean materialist 1820 Thomas Jefferson Existentialism 1841 Soren Kierkegaard [technically, he was a Christian] Darwinian 1859 Nihilism 1882 Friedrich Nietzsche Carnotian 1906 H.G. Wells | Quote: “He was a practical electrician fond of whiskey, a heavy, red-haired brute with irregular teeth. He doubted the existence of a Deity but accepted Carnot’s cycle, and he had read Shakespeare and found him weak in chemistry.”  Second law Partetian (Ѻ) 1914 Lawrence Henderson, Bernard DeVoto Gibbsian 1935 Lawrence Henderson Marxist-Leninist atheism (Ѻ) 1917 A "materialist" advocating understanding of nature Vladimir Lenin Meaninglessness 1937 Aldous Huxley Second law Existential nihilism (Ѻ) 1943 Jean-Paul Sartre (Ѻ), Martin Heidegger, Jacques Derrida Objectivism 1957 Ayn Rand (Ѻ) Purposelessness 1986 Peter Atkins (Ѻ) Second law Pointlessness 1988 Steven Weinberg (Ѻ) Pastafarianism (Ѻ) 2005 A [physics] creationism-parody
Ofer Po (Ѻ)
Goetheanism 2011 A one nature [physicochemical] humanism mixed with physicalism mixed with materialism belief system Libb Thims (Ѻ) Atomic theory
Many will vacillate on the question of atheism for "fear of death", as some have commented, hence maintaining some type of residual theistic belief system (e.g. spiritualism).
“Atheism is so senseless and odious to mankind that it never had many professors.”— Isaac Newton (c.1700), A Short Scheme of the True Religion (Ѻ) (Ѻ)
“The atheists are for the most part impudent and misguided scholars who reason badly, who, not being able to understand the origin of evil, and other difficulties, have recourse to the hypothesis of the eternity of things and of inevitability.”— Voltaire (c.1780) 
“Atheists express their rage against god although in their view he does not exist.”— C.S. Lewis (1950) (Ѻ)