This is a tough question. At school they tell you it is MERRING (movement, excretion, respiration, reproduction, irritability, nutrition and growth), but I do not think that is a very good description.
If it were up to me, I would try and define life in terms of information. You can think about DNA as being a way of transferring information from one generation to another (some life does not use DNA, but there is always an equivalent). Our DNA contains information from many species that preceded humans in our evolution, for example. If your DNA degrades and loses the ability to copy and preserve that information then you age and eventually are no longer alive.
So I would define life as something like a system for acquiring information about the environment and maintaining and copying that information going into the future.