The English word atheist descends from ancient Greek ἄθεος atheos (a- 'without' + theos 'god'). In the oldest texts where it appears, it meant 'impious' or without the favor of the gods. In later texts, it also takes on the meaning of not believing in the gods (Whitmarsh 2021).
Atheist appears in English in the 16th century, followed in the 17th century by deist and then theist. Atheism, deism, theism, and agnosticism each appear in English after their -ist forms (Oxford English Dictionary).
Flew (1972) defines atheism as the absence of belief in gods. Negative atheists don't affirm theism. Positive atheists deny theism. Agnostics deny knowing whether God exists. While agnostics affirm the intelligibility of theism, some negative atheists don't.
Smith (1989) also defines atheism as the absence of belief in gods and distinguishes between explicit atheists who have considered the arguments for God and found them wanting and implicit atheists who have not seriously considered the arguments for theism (because they are unaware, indifferent, and so on). Explicit atheists may deny God's existence or merely withhold belief in God. Agnosticism is a separate stance about what humans can know. Agnostic atheists deny that humans can know whether gods exist. Agnostic theists believe in God and only deny that humans can know God's nature.
Durkin, Philip, and Michael Ruse. 2021. “Linguistic History of the Terms ‘Atheism’ and ‘Atheist.’” Chapter. In The Cambridge History of Atheism, edited by Stephen Bullivant, 11–13. Cambridge: Cambridge University Press.
Flew, Antony. 1972. “The Presumption of Atheism.” Canadian Journal of Philosophy 2 (1): 29–46. http://www.jstor.org/stable/40230372.
Smith, George H. 1989. Atheism: The Case against God. Buffalo, N.Y: Prometheus Books.
Whitmarsh, Tim, and Michael Ruse. 2021. “Ancient Greece.” In The Cambridge History of Atheism, edited by Stephen Bullivant, 84–99. Cambridge: Cambridge University Press.