Source.
This has been kicked around by biologists for millenia. What a pointless question. How can we define something when we only know of one inhabited world to draw definitions from. That's like defining language when you only know one letter of the alphabet, which in any case would make writing up such a definition rather challenging.
Hhh.
H hhh hh hhhh hh h hhhh hh hhhhhhh hh
hhh hhhhh. Hhhh h hhh hhhhh h hhhhh h
hhhh..........
Everything is measured or described in relative terms. If you don't have any reference points you're just lost. The same problem arises with 'intelligence'. Ask a psychologist what intelligence is and he'll balk. Corner him with a broken bottle and you will get the typical reply that 'Intelligence is what intelligence tests measure'. Psychologists routinely throw up such zen-like answers to life's questions. In the article at Space.com the dilemma of pinning down life and pigeonholing it is spelled out. Fire reproduces and breathes for example. Many things are borderline. Perhaps we should just stop splitting hairs because every rule has its numerous exceptions and life undoubtedly arose from inorganic matter anyway. Where in the evolutionary scale do you take a ruler, draw a line and say 'Ok, everything above is robotic chemicals and this guy here is the ancestor of all life on earth.' How is he more deserving than the squishy proto-proto-protozoa that divided into him/her/it/whatever.
We have two hemispheres to our brain for good reason. The answers to some questions can't be engraved in concrete. Like abstract terms they are imagined and only serve to fill a gap in complex calculations. They require an artists subjective intuition. We will likely know life on other worlds when we see it, and if we aren't certain, then it probably won't matter much anyway.