Agnosticism literally means 'Without knowledge' and usually refers to a position on the existence of a deity, but it can be applied to almost any claim. It is sometimes used in relation to things we don't know enough about to form an opinion, or it may be applied to subjects that are inherently unknowable, but it is most often used in reference to claims to the existence or otherwise of a god. It is often assumed - especially by believers - to be a midway point between a believer and an atheist, however the word was coined by the English biologist Thomas Henry Huxley in 1869, and he said "It simply means that a [person] shall not say [they] know or believe that which [they] have no scientific grounds for professing to know or believe."
For the scientist Huxley, the term referred to things that cannot be known, things that are beyond scientific examination or understanding. This would apply to any supernatural claim that is not open to investigation. However, for a skeptic, something that manifests in reality should, in principal, be open to investigation, and if it is not then the null-hypothesis would conclude that it does not exist (though always open to further evidence being discovered). Carl Sagan in The Demon Haunted World gave the example of an invisible pink dragon leaving no trace of its existence; on what grounds can we determine it exists or not if it does not exhibit any signs of it?
Agnosticism is not dichotomous to atheism, nor should it be seen as some half-way point between belief and non-belief. The opposite of Agnostic is gnostic: 'with knowledge'. One can be an agnostic theist (one who believes there is a god but admits they cannot know it), a gnostic atheist (knows there is no god), agnostic atheist (does not believe there is a god but does not claim to know there is not one) or a gnostic theist (knows there is a god). It would generally depend on the claim being made.