Life is the property that distinguishes "objects", or "things" from animals, plants, or anything else that we call "living".
It is easy to recognise (most of the time!) but less easy to describe.
We can describe a number of observable processes which are characteristic of life, but not all are necessary for something to be alive: movement, growth, reproduction, ability to respond to stimuli, respiration, excretion and nutrition.
The main characteristic of life is the ability to self-organise to become more complicated than the surrounding environment, and to sustain oneself in that degree of organization, over time, whilst constantly exchanging materials with the environment.
Thus a cell is alive, whilst a crystal - of similar size and definitely having more complexity than its surroundings - is not.