For me, thethinkfactory comes closest to defining agnosticism.
T. H. Huxley, a friend of Darwin, wrote his own definition. The following is a little of what he felt, as a scientist, defined agnosticism:
Agnostic definition
http://azaz.essortment.com/agnosticdefinit_rmak.htm
Thomas Henry Huxley (1825 1895) came up with the
word agnostic while searching for a term to
describe his own beliefs. He did not consider
himself an atheist, a theist, a pantheist; a
materialist or an idealist; [nor] a Christian and
while he had much in common with freethinkers, he
wanted a term to describe himself more accurately.
His difference with the people who gave themselves
the above labels was that he did not feel certain
of his knowledge- or gnosis- that he had
successfully solved the problem of existence.
The essential problem was that Huxley believed the
problem was unsolvable. And thus far, despite the
existence of famous thinkers like Emmanuel Kant
and David Hume who philosophically agreed with him
on the matter, there wasnt a name for someone who
believed you could never know the source of, nor
reason for existence