@housby,
To start with, 'a definition' is to explain one thing in terms of another. So in the broadest sense, reality is not really definable. So as Kethil says, in some way it is a trick question.
But I could say, in a dictionary sense, 'reality is that which is not illusory' or 'is the absence of delusion or false judgement' - as some have noted. This would be a useful definition most of the time. It would be a satisfactory conventional definition.
But there is something else to consider. In many streams of classical philosophy, and certainly in many religious and spiritual philosophies, 'reality' is that which is 'obscured by ignorance'. In this sense, 'reality' is the vision of the world from the viewpoint of one who is 'saved' or 'gone beyond' or 'free in this life' ('jivanmukti'). This is perhaps also the gist of Plato's metaphor of the Cave, in which he says that most mortals are unknowingly trapped in a shadowy realm, from which the Philosopher, by adherence to the philosophic life and exercise of reason (in the very special sense that Plato understood that word) was able to ascend into the light of day (=reality in the higher sense).
Now from the viewpoint of the traditional philosophies, what most people call 'reality' is actually illusory in some respects. I think the broadest sense in which this is true according to the traditions is that our perception of reality is conditioned by our largely unconscious sense of 'I' and 'mine'. Certainly in Buddhism and the Indian religious philosophies generally, this sense of separate self-hood, which is the common lot of man, is understood to be illusory - 'maya' in the Hindu traditions or 'samsara' in Buddhism. But of course it is very hard to apprehend what they mean by this, because it is obviously difficult to form a picture from outside of it, as it were, as to all intents and purposes it is all that we know - according to them, we are situated within it.
However all of this is in the area of 'traditional studies', 'comparitive religion, and 'philosophy of religion', not really in the area of philosophy of language, or modern Western philosophy.