Definition of "Atheist" from a few different sources:
Merriam-Webster OnLine
atheist: one who believes that there is no deity
atheism:
1 archaic : UNGODLINESS, WICKEDNESS
2 a : a disbelief in the existence of deity b : the doctrine that there is no deity
Cambridge Dictionary of American English
atheist: someone who believes that God does not exist
atheism: the belief that God does not exist
Oxford English Dictionary 2nd Ed. 1989
Atheist:
1. One who denies or disbelieves the existence of a God.
2. One who practically denies the existence of a God by disregard of moral obligation to Him; a godless man.
B. attrib. as adj. Atheistic, impious.
The American Heritage Dictionary of the English Language, Fourth Edition. 2000.
atheist: One who disbelieves or denies the existence of God or gods.
atheism:
1a. Disbelief in or denial of the existence of God or gods. b. The doctrine that there is no God or gods.
2. Godlessness; immorality.
1913 Webster's Revised Unabridged Dictionary
atheist:
1. One who disbelieves or denies the existence of a God, or supreme intelligent Being.
2. A godless person. [Obs.] Syn. -- Infidel; unbeliever. See Infidel.
atheism:
1. The disbelief or denial of the existence of a God, or supreme intelligent Being.
2. Godlessness.
Merriam-Webster OnLine
atheist: one who believes that there is no deity
atheism:
1 archaic : UNGODLINESS, WICKEDNESS
2 a : a disbelief in the existence of deity b : the doctrine that there is no deity
Cambridge Dictionary of American English
atheist: someone who believes that God does not exist
atheism: the belief that God does not exist
Oxford English Dictionary 2nd Ed. 1989
Atheist:
1. One who denies or disbelieves the existence of a God.
2. One who practically denies the existence of a God by disregard of moral obligation to Him; a godless man.
B. attrib. as adj. Atheistic, impious.
The American Heritage Dictionary of the English Language, Fourth Edition. 2000.
atheist: One who disbelieves or denies the existence of God or gods.
atheism:
1a. Disbelief in or denial of the existence of God or gods. b. The doctrine that there is no God or gods.
2. Godlessness; immorality.
1913 Webster's Revised Unabridged Dictionary
atheist:
1. One who disbelieves or denies the existence of a God, or supreme intelligent Being.
2. A godless person. [Obs.] Syn. -- Infidel; unbeliever. See Infidel.
atheism:
1. The disbelief or denial of the existence of a God, or supreme intelligent Being.
2. Godlessness.
Intelligence is the only true moral guide...