[Definition] Agnostic

Agnosticism is considered to be a middle ground between the believer (Theist) and the non-believer (Atheist).
Subtle differences occur in the definition of Agnostic when compared to the other two.
While Theism and Atheism focus their claims on the belief, Agnosticism aims at simply claiming that the impossibility of any of the other two terms lays in the impossibility of obtaining knowledge to prove either atheism or theism to be true or false.

In other words, Agnostics claim that because we cannot prove the existence or the nonexistence of any god, we cannot assert its existence or claim any belief; we do not know.

Here are some types of Agnosticism:

Strong Agnosticism:
A strong agnostic will claim that the truth about divine existence is impossible to find out. No one will ever know.

Weak Agnosticism:
Weak agnostics will claim that we cannot know about divine existence, but if someday some new knowledge comes to light and supports the existence of god(s), we might.

Agnostic Atheism:
Agnostic Atheists share the view of those who do not claim to know of the existence of any divine beings (gods), and do not believe in any due to lack of proof. This is where Agnosticism and Atheism intercept.

Agnostic Theism:
In the opposite side of the Agnostic Atheists, the Agnostic Theists intercept Theism. They share the view of those who do not claim to know of the existence of any deity, but still choose to believe in such an existence.

In these ways, Agnostics tend to stay away from objective claims about divine beings, with the exception of Agnostic Theists (who are Theists nonetheless). They will stop at the fact that proof is not around for us to base our claims on.

More on Agnosticism here.