atheist (definition)

atheist

noun  athe·ist \ˈā-thē-ist\

Simple Definition of atheist

: a person who believes that God does not exist

Source: Merriam-Webster's Learner's Dictionary

Full Definition of atheist

:  one who believes that there is no deity

atheistic  \ˌā-thē-ˈis-tik\ or atheistical play \ˌā-thē-ˈis-ti-kəl\ adjective

atheistically \-ti-k(ə-)lē\ adverb

Editor's note: How agnostic Differs from atheist 

Many people are interested in distinguishing between the words agnostic and atheist. The difference is quite simple: atheist refers to someone who believes that there is no god (or gods), and agnostic refers to someone who doesn’t know whether there is a god, or even if such a thing is knowable. This distinction can be troublesome to remember, but examining the origins of the two words can help.

Agnostic first appeared in 1869, (possibly coined by the English biologist Thomas Henry Huxley), and was formed from the Greek agnōstos (meaning "unknown, unknowable"). Atheist came to English from the French athéisme. Although both words share a prefix (which is probably the source of much of the confusion) the main body of each word is quite different. Agnostic shares part of its history with words such as prognosticate and prognosis, words which have something to do with knowledge or knowing something. Atheist shares roots with words such as theology and theism, which generally have something to do with God.

Source: Merriam-Webster
http://www.merriam-webster.com/dictionary/atheist