A living organism is self replicating, evolving and self-regulating interactive system capable of responding, to external stimuli. Growth, reproduction, nutrition, excretion, etc. are various characteristics of living beings. But most of these characteristics cannot be termed as defining features of life forms.

Defining features of life form are: Growth is defined as an increase in mass. Plants grow throughout their life, while most of the animals grow up to a certain age only. But some non-living things also grow in mass, e.g. cloud or snowball. So, growth cannot be termed as the defining feature of life.

Reproduction is the process by which a living being produces its own kind. Majority of organisms reproduce sexually, while many others reproduce asexually; by various methods. On the other hand, there are many living beings which do not reproduce at all, e.g. the worker bee in a beehive. But we consider it as a living being, So, reproduction  cannot be termed as an all inclusive defining feature of life. As non-living things cannot  reproduce; so, reproduction can be termed as a defining feature of life (with certain exceptions).

etabolism involves various biochemical processes during which various molecules are formed and many others get broken down. Metabolic  eaction cannot happen in a cellfree system. While some in-vitro reactions have been tried out in laboratories but they  cannot be termed as real metabolisms. So, metabolism is one of the defining features of life. 

Structural organization which begin with cellular organization is another character of living beings. Some molecules come together and work in perfect synchronization to form a cell organelle and various cell organelles come together to form a cell. Eventually, cells end up forming the tissue; which become organs and finally the whole organism is formed. Cells are altogether different than their constituent organelles; in terms of characters and functions. Similarly; a tissue is entirely different than its component cells; in terms of functions. Similar demarcation and hierarchy of organization is not possible in non-living things. Hence, cellular organization is a defining feature of life.
External stimuli can reach in various forms inside a living organism and the living being responds to each stimulus in a different way.

This can be termed as consciousness to the environment. Such a consciousness is not possible in non-living things. Hence, response to external stimuli is a defining feature of life.