There are two in-use definitions of the word atheist:

1.) A person who lacks belief in a god or gods. People who use this definition categorize atheists as either negative (or implicit or weak) atheists or positive (or explicit or strong) atheists. Negative atheists, while they don't believe in a god, do not positively assert that no gods exist. Positive atheists, however, do.

2.) A person who believes that no god or gods exist.

Those who consider themselves atheists (who are usually positive atheists) tend to define 'atheist' using the former definition, and those who believe in a god or gods tend to define 'atheist' using the latter. In both cases, this seems to be a demagogic practice intended to classify either as many or as few people as atheists as possible. Negative atheists are usually referred to as agnostics.

While neither definition of atheism entails any personality traits of atheists or of those who do believe in a god or gods (sometimes referred to as 'theists'), both atheists and theists tend to believe that certain traits apply to all atheists/theists. Common myths about atheists include: all atheists are arrogant, immoral, rebellious, and/or intolerant; all atheists really believe in (the Christian) God and are simply in denial; all atheists are depressed and believe...