For the sake of clarity, precision and dialogue I have no problem with defining "atheism" as a lack of belief in God/Goddess/gods/goddesses. In the European philosophical and theological tradition of the last few hundred years atheism was defined as a belief in the non-existence of gods. Only in recent years has it been defined as a lack of belief in gods. It was within that European tradition that atheist crusader Charles Bradlaugh first precisely defined atheism as a lack of a belief in gods in his "A Plea for Atheism" (1864). He had noted elsewhere that "no position is more continuously misrepresented" than atheism. He stated: "Atheism is without God. It does not assert no God." It was probably Thomas Henry Huxley who first to coined the term "agnostic" in 1869.
Modern atheists distinguish between positive atheism and negative atheism (also called strong and weak atheism). Positive atheism is the positive belief that there are no gods. Negative atheism is merely the lack of a belief in gods. Depending on one's perspective, negative atheism can be seen as a form of agnosticism or agnosticism a form of negative atheism. While some positive atheists claim they can disprove ALL gods, knowledgeable positive atheists deny that one must be able to prove the non-existence of all gods and supernatural entities (even if one can disprove some). According to some estimates there have been over 2,500 different gods and supernatural entities believed in and/or seriously described and recorded in human history. To disprove all of them would be an enormous inductive task. But even if, hypothetically, one were successful at disproving all those that does nothing to address unknown, unrevealed or un-thought of supernatural entities. In many cases it's very difficult to prove universal negatives. This is especially true when it comes to supernatural entities because they have a wide variety of claimed attributes, powers, characteristics and personalities. For example, one can easily disprove the existence of gods that actually live on Mount Olympus. But what if Mount Olympus is symbolic of something spiritual and immaterial? When it comes to atheists, for all they know the finite god Zeus does exist but doesn't literally and physically rule from Olympus.
In recent years George H. Smith's book Atheism: The Case Against God was one of the most influential books to disseminate and make popular the definition of atheism as merely a lack of belief in God/Goddess/gods/goddesses.
Here's a link to an article by Smith on Defining Atheism
It is only ignorant atheists who claim that atheism only ever meant a lack of belief in gods. The following will be a list of definitions from reputable (sometimes even authoritative) sources which demonstrate that atheism can mean and has (at least in the past) meant a belief in the non-existence of gods. Some of these sources are contemporary rather than antiquated works. Apparently, these sources are still influenced by the European philosophical and theological tradition I mentioned earlier.
"According to the most usual definition, an atheist is a person who maintains that there is no God, that is, that the sentence "God exists" expresses a false proposition." - Paul Edwards in Encyclopedia of Philosophy Volume I page 175
"Atheism is the view that there is no God." - Internet Encyclopedia of Philosophy
" 'Atheism' means the negation of theism, the denial of the existence of God." - Stanford Encyclopedia of Philosophy
"Atheism is ostensibly the doctrine that there is no God." -- Oxford Companion to Philosophy (2005)
"Atheism (from Greek: a + theos + ismos "not believing in god") refers in its broadest sense to a denial of theism (the belief in the existence of a single deity or deities)."- New World Encyclopedia
"atheism, in general, the critique and denial of metaphysical beliefs in God or spiritual beings. As such, it is usually distinguished from theism, which affirms the reality of the divine and often seeks to demonstrate its existence. Atheism is also distinguished from agnosticism, which leaves open the question whether there is a god or not, professing to find the questions unanswered or unanswerable."- Encyclopædia Britannica
"ATHEISM (from Gr. ἀ-, privative, and θεός, God), literally a system of belief which denies the existence of God. The term as generally used, however, is highly ambiguous." - Encyclopædia Britannica 11th edition
“Atheism is the deliberate, definite, dogmatic denial of the existence of God. It is not satisfied with appropriate truth or relative truth, but claims to know the ins and outs of the game being quite clearly the absolute denial of the absolute.” - Etienne Borne
Atheism, from the Greek a-theos ("no-god"), is the philosophical position that God doesn't exist. It is distinguished from agnosticism, the argument that it is impossible to know whether God exists or not (Academic American Encyclopedia).
Atheism, system of thought developed around the denial of God's existence. Atheism, so defined, first appeared during the Enlightenment, the age of reason (Random House Encyclopedia-1977).
Atheism (from the Greek a-, not, and theos, god) is the view that there are no gods. A widely used sense denotes merely not believing in God and is consistent with agnosticism. A stricter sense denotes a belief that there is no God, the use has become the standard one (Cambridge Dictionary of Philosophy-1995).
Atheism is the doctrine that there is no God. Some atheists support this claim by arguments, but these arguments are usually directed against the Christian concept of God, and are largely irrelevant to other possible gods (Oxford Companion to Philosophy-1995).
Atheism is disbelief in God (Introduction to Philosophy, Perry and Bratman, Oxford University Press-1986).
Atheism from the Greek a (not) plus theos (god). The doctrine of disbelief in a supreme being (Dictionary of Philosophy and Religion, William Reese, HumanitiesPress-1996).
Atheism (Greek, a- [private prefix] + theos, god) is the view that there is no divine being, no God (Dictionary of Philosophy, Thomas Mautner, Editor-1996).
Atheism is the belief that God doesn't exist (The World Book Encyclopedia-1991).
Atheism, Greek atheos-Disbelief in, or denial of, the existence of God (Oxford English Dictionary-1989)
Atheism, commonly speaking, is the denial of God. Theism (from the Greek theos, God) is belief in or conceptualization of God, atheism is the rejection of such belief or conceptualization.In the ancient world atheism was rarely a clearly formulated position (Encyclopedia Americana-1990).
Atheism, the critique and denial of metaphysical beliefs in God or spiritual beings. Atheism is to be distinguished from agnosticism, which leaves open whether there is a god or not, professing to find the question unanswerable, for the atheist, the non-existence of god is a certainty (The New Encyclopedia Britannia-1993).
According to the most usual definition, an atheist is a person who maintains that there is no god…(rejects eccentric definitions of the word) (The Encyclopedia of Philosophy-1967).
Atheism is the doctrine that God does not exist, that belief in the existence of God is a false belief. The word God here refers to a divine being regarded as the independent creator of the world, a being superlatively powerful, wise and good (Encyclopedia of Religion-1987).
Atheism (Greek and Roman): Atheism is a dogmatic creed, consisting in the denial of every kind of supernatural power. Atheism has not often been seriously maintained at any period of civilized thought (Encyclopedia of Religion and Ethics-Vol II).
Atheism is the rejection of theism: a-theism. Atheists maintain some or all of the following claims: that theism is false; that theism is unbelievable; that theism is rationally unacceptable; that theism is morally unacceptable. G. Oppy, "Arguments for Atheism," S. Bullivant & M. Ruse, eds. The Oxford Handbook of Atheism (OUP, 2014), 53.
Some quotes taken HERE
See this YouTube audio with William Lane Craig
Reasonable Faith podcast with William Lane Craig on the definition of "atheism"
Book Reviews of Recent Atheist Authors
by Christian Apologists
by Christian Apologists