The lack of theistic belief. Not believing in God or other deities. From the Greek etymology 'a-' meaning 'without' and 'theism' meaning 'belief in a deity'.
Often incorrectly asserted that it comes from the Greek 'atheos' (ἄθεος) meaning 'godless, immoral' and originally a derogatory word, as 'theism' is derived directly from 'theos' (θεός) meaning 'God'. This is false. The word 'atheist' was coined in the 18th century and way originally pronounced with emphasis on the 'a-'. The connection between the new word and the Greek word has been the basis behind many dictionaries in the last few years changing their definition of atheism to:-
"The doctrine that there is no God".
This has caused the word 'agnostic' meaning 'without absolute knowledge' to be used incorrectly in place of 'atheist' in the last decade. With recent campaigns and the increasing number of vocal atheists, this is being slowly changed back.