Life is not a simple word that we can define in single definition. For someone who's unhappy for him life is curse and someone who is happy, life is gift to him. In science language we can define it in a definition but in reality life is beyond that definition. It is full of joy, sorrow, excitement and many more feelings. It can convert your happiness into sorrow within a minute and that's why life is unpredictable also.
We have heard that line that we live only ones and that's true about life. You never know what will happen next. Life can be meaningless to someone who never lived it.