16th Century

An Athe(os)-ist is someone who believes that gods do not exist. In the 1570s, the French pulled the word “atheos” (no/not/without gods) out of Greek antiquity and added the suffix, “iste” (someone who believes, a believer), which became “someone who believes no gods exist”. The English followed suit.

atheist (n.)
1570s, from French athéiste (16c.), from Greek atheos “without god, denying the gods; abandoned of the gods; godless, ungodly,” from a- “without” + theos “a god” (see theo-).

John Florio, A World of Words (1598)
Atèo, Atheo, Atheista, an atheist, a miscreant, godles, one that thinkes there is no god.

17th Century

Randle Cotgrave, A Dictionary of the French and English Tongues (1611)
Athée: m. An Atheist; one that beleeues there is no God.

John Bullokar, An English Expositor (1616)
Atheist. He that wickedly beleeueth there is no God, or no rule of Religion.

Henry Cockeram, English Dictionary (1623)
Atheist. That thinks there is no God, or rule of religion.

Thomas Blount, Glossographia or a Dictionary (1656)
Atheist ( from the Gr ἄδεος. id est Sine Deo, godless) he that beleeves there is no God or rule of Religion, and that the soul dies with the body.

theist (n.)
1660s, from Greek theos “god” (see theo-) + -ist. The original senses was that later reserved to deist: “one who believes in a transcendent god but denies revelation.” Later in 18c. theist was contrasted with deist, as believing in a personal God and allowing the possibility of revelation.

^The word “theist” finally came into existence. The English pulled the word “theos” (god) out of Greek antiquity and added the suffix, “ist” (someone who believes, a believer), which became “someone who believes god exist”. Prior to this, for almost a full century, there was no word “theist” to attach an “a” prefix to.

18th Century

John Kersey the younger, A New English Dictionary (1702)
Atheism, the Opinion of
An Atheist, who denies the Being of a God.

An Universal Etymological English Dictionary, Nathan Bailey, R. Ware, 1756
ATHEIST (from Gr. without God) one that denies the exiftence of God.

A dictionary of the English language., Samuel Johnson, 1768
A’the-ist, f. One that denies the existence of a God.

19th Century

A primary-school pronouncing dictionary of the English language, Noah Webster, William Greenleaf Webster, J.B. Lippincott & Co., 1857
A’the-ist, n. one who denies the existence of a God.

Atheism in pagan antiquity by Drachmann, A. B. (Anders Björn), 1860-1935

ATHEISM and atheist are words formed from Greek roots and with Greek derivative endings. Nevertheless they are not Greek ; their formation is not consonant with Greek usage. In Greek they said atheos and atheotes ; to these the English words ungodly and ungodliness correspond rather closely. In exactly the same way as ungodly, atheos was used as an expression of severe censure and moral condemnation ; this use is an old one, and the oldest that can be traced. Not till later do we find it employed to denote a certain philosophical creed ; we even meet with philosophers bearing atheos as a regular surname. We know very little of the men in question ; but it can hardly be doubted that atheos, as applied to them, implied not only a denial of the gods of popular belief, but a denial of gods in the widest sense of the word, or Atheism as it is nowadays understood.

20th Century

Atheism: The Case Against God, by George H Smith, 1973

“First, there is the familiar sense in which a person is an atheist if he maintains that there is no God, where this is taken to mean that “God exists” expresses a false proposition. Secondly, there is also a broader sense in which a person is an atheist if he rejects belief in God, regardless of whether his rejection is based on the view that belief in God is false.”
“This immediately raises the question of agnosticism, which has traditionally been offered as a third alternative to theism and atheism.”
“An agnostic is not an atheist. An atheist denies the existence of God; an agnostic professes ignorance about His existence. For the latter, God may exist, but reason can neither prove nor disprove it.”
“Notice that agnosticism emerges as a third alternative only if atheism is narrowly defined as the denial of theism.”

The Presumption of Atheism, by Antony Flew, 1984

In this interpretation an atheist becomes: not someone who positively asserts the non-existence of God; but someone who is simply not a theist. Let us, for future ready reference, introduce the labels ‘positive atheist’ for the former and ‘negative atheist’ for the latter.
The introduction of this new interpretation of the word ‘atheism’ may appear to be a piece of perverse Humpty-Dumptyism, going arbitrarily against established common usage. ‘Whyever’, it could be asked, ‘don’t you make it not the presumption of atheism but the presumption of agnosticism?’

21st Century

Now, although a broader definition has gained some popularity over the past 30 years, it is still not the only definition. The narrow definition is still usually included, and the broader definition is still sometimes not included. The broad definition is most popular in atheist circles.

“one who believes that there is no deity”

“a person who believes that God does not exist”

“someone who believes that God does not exist”

“a person who believes that God does not exist”

“someone who believes that God does not exist”

“one who believes that there is no God or gods.”

“someone who denies the existence of god”