'Living' means a thing which can grow, reproduce and has cellular organisation. A living thing is also aware of its surrounding. These unique characteristics make a living thing different from non-living things.
Four defining features of life forms are:
(a) Growth: It is the permanent increase in cell number and size of the organism. It involves cell division. Dead organisms do not grow.
(b) Reproduction: The ability of an organism to produce progeny, more or less similar to itself, is called reproduction. Two main modes of reproduction found in organisms are asexual reproduction and sexual reproduction.
(c) Metabolism : The sum total of all the chemical reactions occurring in the body of an organism is metabolism. No non-living object exhibits metabolism.
(d) Cellular organisation: The body of all living beings consists of one or many cells, therefore cell is known as fundamental unit of life.