Thomas Huxley invented the word "agnosticism" who defined it like this:
Agnosticism is not a creed but a method, the essence of which lies in the vigorous application of a single principle... do not pretend conclusions are certain that are not demonstrated or demonstrable. LINK.But Bertrand Russell suggested a different definition:
An atheist, like a Christian, holds that we can know whether or not there is a God. The Christian holds that we can know there is a God; the atheist, that we can know there is not. The Agnostic suspends judgment, saying that there are not sufficient grounds either for affirmation or for denial. At the same time, an Agnostic may hold that the existence of God, though not impossible, is very improbable; he may even hold it so improbable that it is not worth considering in practice. In that case, he is not far removed from atheism. LINK.