What is meant by living ? Give any four defining features of life forms.

Explanation

An organisms said to be living, if is shows certain characteristic feature that makes it different from non-living things.
The following features are defining features of life forms :
(1) Growth : Living things grow by increase in mass and increase in number of individual or cells. In living beings growth is internal.
(2) Reproduction : It is essential to maintain the continuity of life. Many organisms eg. Mules, sterile worker bees, infertile human couples are not able to reproduce while they have all other defining properties of life so they can be called as living.
(3) Metabolism : It is the sum total of reactions occurring inside the body by which living things assimilate energy and use it for growth. 
(4) Response to stimuli : Consciousness and response to stimulus is the defining property of all living organisms.
(5) Cellular organisation : The cells are the structural and functional unit of living organisms.