This web site uses the term "nontheists" to refer to atheists and agnostics. One reason for this is that both "atheist" and "agnostic" are often misunderstood terms. The definitions that are commonly (but not universally) used in the philosophy of religion are as follows:

An agnostic is a person who does not know if a god or gods exist. An agnostic who also believes that it is impossible to know whether or not gods exist is a strong agnostic, otherwise he or she is a weak agnostic. To illustrate the difference between a weak agnostic and a strong agnostic, consider how an agnostic would view the claim of a theist who says that she knows that a god exists. The weak agnostic would not automatically conclude that the theist is unjustified in making this claim, since she may have additional evidence that the weak agnostic does not posses. The strong agnostic, however, believes that the evidence required to make the theist's claim doesn't exist, and therefore the strong agnostic would conclude that the theist does not actually know that a god exists.

An atheist is someone who does not believe in the existence of a god or gods. An atheist who actively believes that gods do not exist is a strong atheist, while an atheist that merely lacks belief in gods is a weak atheist. Many people who identify themselves as agnostics are also weak atheists. Note that since it is perfectly consistent to hold the position that the evidence for the existence or nonexistence of gods is not entirely conclusive (so knowledge is not possible) but points heavily towards the nonexistence of any gods (so a tentative belief in the nonexistence of gods is justified), it is possible to simultaneously be a strong agnostic and a strong atheist.

There are other definitions of atheism and agnosticism that are sometimes used. For example, T. H. Huxley, who first coined the term "agnostic", said:

"Agnosticism, in fact, is not a creed, but a method, the essence of which lies in the rigorous application of a single principle. ... Positively the principle may be expressed: In matters of the intellect, follow your reason as far as it will take you, without regard to any other consideration. And negatively: In matters of the intellect, do not pretend that conclusions are certain which are not demonstrated or demonstrable."