I was having a conversation with a non buddhist about consciousness I was struggling to define some key aspects and it kind of led into not self, but I was looking for a concise simple definition.
Technically, consciousness (viññāṇa) is that aspect of the mind that is aware; it is the qualia of modern Western philosophy.
Colloquially, it is used to refer to the entire mental ensemble that includes sensation (vedanā), recognition (saññā), processing (saṅkhāra), and awareness (viññāṇa). This is because the other three aggregates are always accompanied by awareness. Just as water has colour, temperature, etc., so too, consciousness has sensation and other mental concomitants.
Doctrinally, it is a cornerstone of Buddhist philosophy, which takes an experiential paradigm as the basis of reality (as opposed to the temporospatial paradigm of modern science). Each conscious qualia constitutes an atomic entity in the complex matrix of multi-perceptional reality.
cittena nīyati loko, cittena parikassati.
cittassa ekadhammassa, sabbeva vasamanvagūti.
By the mind the world is led, by the mind is it carried away.
Of the mind, the one thing, all indeed go according to its power.
-- SN 1.62
Consciousness in English has a different semantic range than it has in Pali. Westerners have spent a lot of ink writing about consciousness, so I think there is a nontrivial risk of talking about biscuits (British cookies) and biscuits (American little round bread thingies).
Some of the relevant Pali jargon words are:
viññāṇa- awareness, especially the sort that is like a chain reaction starting with the physical senses of sight, touch, etc that leads to thoughts of other things. Sometimes translated as consciousness. http://en.wikipedia.org/wiki/Vij%C3%B1%C4%81na#Overlapping_Pali_terms_for_mind
The Buddhas teaching on the Five Aggregates will help you perfectly explain the consciousness!
Here is a passage from the link below:
But physical elements by themselves are not enough to produce experience. The simple contact between the eyes and visible objects, or between the ears and sound cannot result in experience without consciousness (Vijnana). The eyes can be in conjunction with the visible object indefinitely without producing experience. The ears too can be exposed to sound indefinitely without producing experience. Only the co-presence of consciousness together with the sense organ and the object of the sense organ produces experience. In other words, it is when the eyes, the visible object and consciousness come together that the experience of a visible object is produced. Consciousness is therefore an indispensable element in the production of experience.
If you do practice formal sitting meditation, you will experientially see this. When you focus on the rising and falling of the stomach, once the mind wanders to a different experience, you no longer are aware of the stomach, this is because you no longer have the physical and mental duality, so there is no experience of the stomach, there is no existence, but..when the mind returns to the stomach, once again you have contact between the mental and physical, and once again the sensation of the stomach is realized! (just a side note: The changing experience is "rebirth/birth)
The Pali scriptures define consciousness as 'sense cognition' or 'sense awareness'. This is the most accurate single definition to use.
'Consciousness, consciousness': Thus is it said. To what extent, friend, is it said to be 'consciousness'? 'It cognizes, it cognizes': Thus, friend, it is said to be 'consciousness.' And what does it cognize? It cognizes 'pleasant.' It cognizes 'painful.' It cognizes 'neither painful nor pleasant.' 'It cognizes, it cognizes': Thus it is said to be 'consciousness.'
And why do you call it 'consciousness'? Because it cognizes, thus it is called consciousness. What does it cognize? It cognizes what is sour, bitter, pungent, sweet, alkaline, non-alkaline, salty, & unsalty. Because it cognizes, it is called consciousness.
And what is consciousness? There are these six classes of consciousness: eye-consciousness, ear-consciousness, nose-consciousness, tongue-consciousness, body-consciousness, mind-consciousness.
The six classes of consciousness should be known.' Thus was it said. In reference to what was it said? Dependent on the eye & forms there arises consciousness at the eye. Dependent on the ear & sounds there arises consciousness at the ear. Dependent on the nose & aromas there arises consciousness at the nose. Dependent on the tongue & flavors there arises consciousness at the tongue. Dependent on the body & tactile sensations there arises consciousness at the body. Dependent on the intellect & ideas there arises consciousness at the intellect.
'Apart from a requisite condition, there is no coming-into-play of consciousness.'
Consciousness, monks, is classified simply by the requisite condition in dependence on which it arises. Consciousness that arises in dependence on the eye & forms is classified simply as eye-consciousness. Consciousness that arises in dependence on the ear & sounds is classified simply as ear-consciousness. Consciousness that arises in dependence on the nose & aromas is classified simply as nose-consciousness. Consciousness that arises in dependence on the tongue & flavors is classified simply as tongue-consciousness. Consciousness that arises in dependence on the body & tactile sensations is classified simply as body-consciousness. Consciousness that arises in dependence on the intellect & ideas is classified simply as intellect-consciousness
Consciousness is simply ability to know or, more complicated, it's property to have object. Pali commentaries define consciousness (citta) in three ways. Agent definition - citta is what cognizes an object (arammanam cinteti ti cittam). Instrument definition - by means of which mental factors (cetasika) cognize the object. Activity definition - nothing other than cognizing (or knowing) the object. Third definition is considered to be ultimate definition, while first two is provisional definitions, all of them are supposed to refute view that these functions are performed by Self.
Probably consciousness could be hard to define in mechanistic-materialistic view (or standpoint), it's because such view eliminate consciousness. So, how to define thing that is fundamentally eliminated? To correct this we should recognize that consciousness is of fundamental (phenomenal) nature and does not need to be reduced to matter or derived from matter. For all of us consciousness is basic fact (phenomenon).
There is no single definition of consciousness, but what seems to be common to all, at least implicitly, is the existence of an internal model of the "external" world, and sensory input to inform the model. Humans have several kinds of sensory input, and their internal models usually consist of a field of different densities (objects with different "hardnesses, e.g. steel, wood, water) and with varied luminance, along with sound, flavor, scent, and so on. Bacteria have very different consciousnesses, built almost solely on chemical concentration gradients (which way to the sugar?).