What is a fact? What kind of object, if it is indeed such a thing, is it? I have read a lot of stuff that say "it is a fact that... ", but I have never seen a definition of fact or what kind of thing a fact is.
I would look at the Stanford Encyclopedia of Philosophy entry on Facts: http://plato.stanford.edu/entries/facts/. There are lots of articles and monographs in recent metaphysics (and in work on Russell, Ramsey, Wittgenstein, etc.) that focus on this topic, and some important ones show up in the bibliography.
One possible approach to the philosophical term fact is Wittgenstein's Tractatus Logico-Philosophicus. It begins:
- The world is all that is the case.
1.1 The world is the totality of facts, not of things.
1.11 The world is determined by the facts, and by their being all the facts.
1.12 For the totality of facts determines what is the case, and also whatever is not the case.
1.13 The facts in logical space are the world.
1.2 The world divides into facts.
Wittgenstein does not give a definition of the term fact. But one can try to derive the meaning of the different terms: Proposition 1.1 states that the basic entities in his ontology are not single things but relations between things, which he calls facts. Considered this way, Wittgenstein takes thing as an undefined basic notion and uses it to define the term fact.
If one calls a possible relation a circumstance (= Sachverhalt, in German), then a fact is a real circumstance (= Tatsache, in German). Hence the real world is singled out from all possible worlds by those circumstances which are facts.
I think the most simple answer would be 'an event or occurrence which most people will never argue about'. Like the sky being blue, or a person with a penis being a man and a person with breasts being a woman, cars being able to drive faster then me on my bike etc.
I would say undisputed facts, but that's never really true because it's everyone's right to disagree and see things differently. And sure, the sky is never always blue, a person with a penis can be a woman and there are cars slower then me on my bike.
But that's the clue, everything is relative. So are facts. Facts can change. For now it's a fact the sun comes up every morning, it will remain a fact until it just doesn't. The same with the black swan; to prove there are only white swans in this world, you need to find a black one.