Depending on who you are, the definition varies.
Scientifically, life happens when cells are active, using up ATP and creating byproducts, such as oxygen and carbon dioxide (depending on whether you are looking at plants or animals).
In regards to sustainability and medical definitions, for there to be actual life the cells must be viable and able to survive in their intended environment for a proper life-cycle (the stages that all things classified as 'alive' go through in order to keep on living). If this does not occur, then often the live cells do not create 'life'.
Religiously there are stricter definitions, since abortion is the easiest issue to pin down, they believe that conception is life, even though at that point the live cells still technically belong to the parents (ovum and sperm). With those two live cells combine (and begin to replicate) into a blastocyst, then it is considered 'life'.
Of course, if you are talking about 'living' and 'having a life', that is a different kettle of fish entirely.
To some, you must be active and enlightened and seeking new pursuits to be truly alive, and to others it is permissible to have a routine, regular, ordinary life.
In most cases life is considered breathing, active, productive creatures within an environment who sustain themselves either through independence or symbiosis, or even as parasites.
So when you ask, "What is life?" you are truly walking into a realm of interpretation that requires several factors to interpret. A more specific question should be submitted to gain a properly specific answer.