A question asked many times by many great people long before you and I could comprehend. Humanity as a whole has been burdened with the gift of a higher plane of thought and intelligence naturally required to live in their world. We've become comfortable to the point where we can just sit there and ponder our own existence.
If I look it up in at dictionary.com defines life as...
1. - the condition that distinguishes organisms from inorganic objects and dead organisms, being manifested by growth through metabolism, reproduction, and the power of adaptation to environment through changes originating internally.
2. - the sum of the distinguishing phenomena of organisms, especially metabolism, growth, reproduction, and adaptation to environment.
3. - the animate existence or period of animate existence of an individual: to risk one's life; a short life and a merry one.
4. - a corresponding state, existence, or principle of existence conceived of as belonging to the soul: eternal life.
5. - the general or universal condition of human existence: Too bad, but life is like that.
Then of course there is the smart ass answer from the popular book series, Hitchhiker's Guide.
Ugh, that just reminds me that I need to catch up the Hitchiker's Guide Series, I'm only half way through The Restaurant at the end of the Universe (Book 2). Yeah yeah, don't yell at me, I'll get on it now. I had no idea there were actually 6 of these books now....
The 6th written by Eoin Colfer after the original 5 that Douglas Adams wrote. So yeah know, I've done the appropriate research. (Read the Wikipedia article).
I'll finish this post on a later date... Maybe.