The ability to think and act of its own accord, whilst analyzing its surroundings, while holding an opinion.
The problem with that is sometimes i wonder whether some people qualify
No offence to the first two answers... plants are alive and cannot move own their own or have thought... i don't believe that micro-organisms have thought either.
M.R.S.N.E.R.G - Movement, Respiration, Sensitivity, Nutrition, Excretion, Reproduction and Growth.
Movement? Thought? Emotion? And when I say that, I also mean in an more artistic way as well.
If it is aware of the world around it, then it is alive
It can respond to stimulus and it can reproduce.
Energy consumption (it eats) and reproduction.
If it breathes, and eats...its alive.