A number of definitions exist for "Reality".\ Reality is what we can get away with - Robert Anton Wilson\ Reality is that which, when you stop believing in it, does not go away. - Philip K Dick\ Broadly speaking, reality is that which is real. This is called the Reality Principle. It is tautologous, meaning it is true under all circumstances. Though it seems trivial at first, it is a useful guiding principle in thinking about reality. Anything relevant to reality is real, anything else can be disregarded, because it ain't real.
A comprehensive theory of reality would have to explain both subjectivity and objectivity and how they relate, both the mental and physical planes and how they relate, both matter and information (ontological hardware and software) and how they relate. Science, because of its presupposition that reality is empirical and law-governed, is on its own insufficient to provide a reality-theory. Reality-theory has to involve science, philosophy, psychology and mathematics/logic.
Christopher Langan, the smartest bouncer in the world, has devised a theory of reality he calls the Cognitive-Theoretic Model of the Universe, the CTMU. http://www.ctmu.org/ gives information about it, including a short abstract in pdf format.\ What's fascinating about Langan's theory is its introduction of a third principle of Happening, an alternative to causal determination on the one hand and randomness on the other. Langan calls this third possibility "self-determinacy". Self-determinacy is seen in the CTMU as a contolling principle which a system has unto itself, independent of external factors. Remember that Aleister Crowley had proffered the theory that everyone has his own 'True Will', which is neither determined nor random, and that Lao-Tzu had similarly described ziran, which means roughly "that which happens of its own accord". Langan's theory captures the insights of Crowley and Lao-Tzu, but develops them more rigorously.
We normally think of information as something separate from matter, like matter is the substrate and the form is imposed upon that. Physics has discarded this distinction, and sees particles as space-time patterns, ripples of information in silence. This totally conforms to the mystic view of reality what you could call the Mind of God. The universe is nothing but information. As Terence McKenna was fond of saying, "The universe is made of language". Langan calls this, in his prosaic language, "infocognitive monism".
Anandavala is a wondrous website compiling the thoughts of a jnan-yogi by the name of John Ringland. For decades, he has singlemindedly pursued the wisdom contained in the psychedelic vision, mathematics, magick and yoga, never letting himself be swayed by the biases of men, never binding himself to a single school and sticking steadfastly to the truth. His theory is similar to Langan's: reality is information-processing. Matter is a more determined sort of information, consciousness is a less determined sort of information, and behind it all is a transcendent realm of zero information.
http://www.unifiedreality.com/ deals with another attempt to provide a comprehensive reality-theory.