This article is from the Atheism FAQ, by mathew firstname.lastname@example.org with numerous contributions by others.
The term 'agnosticism' was coined by Professor T.H. Huxley at a
meeting of the Metaphysical Society in 1876. He defined an agnostic as
someone who disclaimed both ("strong") atheism and theism, and who
believed that the question of whether a higher power existed was
unsolved and insoluble. Another way of putting it is that an agnostic
is someone who believes that we do not and cannot know for sure
whether God exists.
Since that time, however, the term agnostic has also been used to
describe those that do not believe that the question is intrinsically
unknowable, but instead believe that the evidence for or against God
is inconclusive, and therefore are undecided about the issue.
To reduce the amount of confusion over the use of term agnosticism, it
is recommended that usage based on the original definition be
qualified as "strict agnosticism" and usage based on the second
definition be qualified as "empirical agnosticism".
Words are slippery things, and language is inexact. Beware of assuming
that you can work out someone's philosophical point of view simply
from the fact that she calls herself an atheist or an agnostic. For
example, many people use agnosticism to mean what is referred to here
as "weak atheism", and use the word "atheism" only when referring to
Beware also that because the word "atheist" has so many shades of
meaning, it is very difficult to generalize about atheists. About all
you can say for sure is that atheists don't believe in God. For
example, it certainly isn't the case that all atheists believe that
science is the best way to find out about the universe.