What is the real definition of an Atheist ?

Atheism is the lack of belief in deities.

Theism is a large umbrella term referring to a belief in a God or Gods. It comes from the Greek word for God, Theos. A is a Greek negation prefix. So atheism is a broad umbrella term meaning, literally, "no Gods", but is more generally understood as a lack of belief in God or Gods.

It is not a blanket rejection of religion, because some religions are largely atheistic, like many schools of Buddhism, some UU congregations, pantheism (although there's some dispute in that area, I personally identify with pantheism, and agree that the net effect means you can put us in the atheist camp, even if we're willing to equate the natural universe with God.)

Most types of pagans believe in two or more Gods.

Heresy is whatever any group says it is. Usually it's more an "in-group" rather than "out-group" kind of issue. For instance, while anyone not Christian is an unbeliever, it's the differences between Christian groups than counts as heresy. So when trinitarians label unitarians (believers in the trinity vs believers in a single God who has a subordinate son and holy spirit helper) heretics and vice versa, things can get messy.

Gnostics are usually associated with a mystic, symbolic understanding of Christianity, although some claim the traditions reach back further than Christ. It stems from the Greek word "gnosis" which means "knowledge", but in this sense, meant metaphysical knowledge.

Agnostic, therefore, means "not knowledge" or rather "not knowable."

Agnosticism is a positive assertion about the nature of knowledge. It just states matters beyond nature, metaphysical speculations, cannot be asserted as factual. Thus people who BELIEVE there is no God and people who BELIEVE there is one or more, but who are willing to admit that such beliefs cannot be asserted as fact are all agnostics, agnostic atheists and agnostic theists. The closest you could get to a pure agnostic (and the meaning Thomas Huxley intended when he coined the term in the late 1800s) is someone who says that since no facts about God(s) can ever be known and proven, then speculating is a waste of time and energy.

Edit: re: your additional details, see if these folks resonate with you...

http://pantheism.net/

An atheist is someone who doesn't believe in the existence of any deity.

Some atheists are irreligious and others actually do practice a religion. For example, there are some Pagans who are atheists.

Yes. All of the above. (Except for the part about heretics...) Strictly speaking, an ATHEIST is someone, such as IJR, who does not believe in the existence of any deities of any kind. No Supreme Being, no God, no gods, no goddesses, no devil, no Satan, etc.

Greek lesson:

"A" = "without"

"Theos" = "god"

An atheist is without a belief in a deity.

That's it. That's all.

As atheists are individuals, of course, there are some atheists who not only do not believe in a god but also insist there is no god. However, all it takes is no belief in any gods or goddesses to be an atheist.

Theist: One who believes in the existence of a god or gods

Prefix "a-": Without, lacking

Atheist: One without belief in a god or gods

Athiest just means you don't believe in or worship a diety or higher power such as a god.

Quite simply, an atheist is a person who denies the existence of any deity, such as a god.

Lack of belief in Gods or Goddesses

I do not believe in God ...Never will no matter what you say...And never did!!..I do try to keep an open mind...Because seeing no body really knows...There might be that little some thing...We will never know in till then!!!

Someone who lack belief in any god or deity.