If we do not have a defenition for life there can be no test for life, as this in itself could be a definition.

If you can kill it, it was alive.

True, what about viruses, they are on the borderline. A lot of people regard them as life that can be killed, but some say it isnt life. A definition of life still has to be known if you want to know if you have just ceased the function of a very complex mineral (what some people regard viruses as being), or have killed a living organism.

Life is probably reproducing, growing repairing…. taking som kind of nutrient/substance to then become bigger and reproducing.

If it can do that on its own, thats life

In the study of abiogenesis, early living systems need to be able to self-organize (convert energy), reproduce, and evolve; later systems are cellular, with protein-based chemistry and DNA coding.

But that’s terrestrial life…

why just terrestrial…?

membranes developed in the ocean

Terrestrial because we can’t be sure what limitations this environment had on the development of some features.

Is cellularity probably common? Yeah, but it might not be a "deal-breaker" feature if you’re deciding Life or Not Life.

Personally, I think viruses are descendants of some of the earliest terrestrial life, from the precellular period. Definitely Life. But many disagree, for various reasons.

Poke it with a stick??? lol

Doesn’t all life respond to some type of stimulus?

If not, what living organism doesnt respond to some sort of stimulus?

This is intensely interesting, and is the crux of Biology for me.

All the suggestions so far given, while excellent, can be applied to many things or argued against. Think about this.

I refer you all to the work of Lovelock, who asked the same question. "How do we know there is life on Earth?".

In short, and even this has intense criticism, the test for life is simply this: energetic disequilibrium.

Think about the atmosphere of Mars – this is chemically balanced according to the physics that govern the gases (etc.). Now consider Earth’s atmosphere: there should simply not be 21% Oxygen in the atmosphere under ‘normal’ circumstances – its absurd. The chemical is highly reactive and frankly dangerous.

I may appear to be talking nonsense here, but I urge you to consider this thoroughly. "Life fights chaos".

(But I particularly like ‘poke it with a stick’)

Yes but how do you find a "correct" stimulus? I can poke bateria for hours with a stick without seeing any evidence of reaction. But If I had some glucose to their Minimal medium, I will be able to see something.
Now identifying the stimulus that will collect a life like response from alien life form might be hard to identify.

The criterion offered by Kotokeru seems better.

A computer reponds to a stimulus – is that life?

A computer was created by another creature, all it is is lights and clockwork that do what you tell it to do(and not necessarily what you want it to do…)

–At first I was going to say a computer is dependent to a power source, but that doesn’t really work because most creatures are dependent to the sun as a power source, making us all computers!! 😆