Something is alive when it has the ability to change its current dynamic composition through feedback mechanisms with the environment and duplicate its current dynamic composition. One example is a DNA molecule that has genes to regulate, such things as, “cytosine to 5-methylcytosine to 5-hydroxymethylcytosine” changes in its composition and to replicate these changes. Another example is when one is a Christian and converts others. One can be alive in a physical sense and dead in a spiritual sense.
I define it myself, because it seems others define it in too broad a sense. Certain cancer cells seem already dead, long before they are finally destroyed completely. It apparently makes some people happy to try to define most everything as being alive, but life is more and special to me.