It's a tough question too because we really do not understand consciousness, itself a very dynamic and often complex topic. However, generally when one uses the term, they are referring to the effects of some illicit drugs (sorry prgrmdave...gonna be tough to avoid that topic with a question like this, but I completely understand your point), and also meditation or spiritual/religious influences.
The easy answer is that it is mental awareness of being in the present in such a way that is different fundamentally from your normal day-to-day experience. To say "what it feels like," is like asking "what does Parprikash taste like?" Different for everyone...
An altered state of consciousness could result from something as simple as going from your living room into the outdoors. What does it mean to you, zee?
