Definitions of life

Here are 3 practical definitions of life, which are useful in their respective contexts and are also fairly independent from each other.

Medical definition of life: self-sustenance

A physical entity, called an organism, is alive as long as it is able to take care of itself to some large extent. It should be able to acquire and process resources needed to maintain its general state. Life is defined per individual and ends with the death of that individual. Whenever death is being discussed, this is the definition of life that’s being used.

Examples:

Fortunately no human lives were lost during this accident.
The android’s life ended as he attempted to swim across the English Channel.
Humans rarely live longer than 100 years.

Counter-examples:

Rocks are not alive as they don’t regenerate in any way after being altered by external conditions.

Biological definition of life: self-replication

In science, life is defined for a set of organisms with a common ancestor. It is the ability for one or a few of such organisms to produce similar organisms with the same ability to reproduce.

Examples:

All life observed today on Earth derives from a common ancestor.
Some wonder whether life existed on an asteroid before being brought onto Earth.
Life on Earth is at least 3.5 billion years old.

Chemical reactions, nuclear reactions, and dominoes collapsing are typically not normally considered life, although they somewhat fit the definition above. Their reproduction can be seen as a spontaneous reaction of the form:

Source of Matter and Energy + Parental Energy → Unusable Matter + more Parental Energy

Technological definition of life: self-adaptation

In this definition, physical entities that can improve the efficiency with which they deal with their environment are said to be life-forms. This includes all organisms and machines capable of learning. This excludes evolution in the traditional biological sense where random mutations generated in a parent organism take effect only in the children, where children constitute distinct organisms. If the mutated children were considered part of the same organism as the parents, which we’d call a colony, then it would be fair to treat this trial-and-error internal evolution as a mechanism for self adaptation.

Examples:

Dogs learn to identify and please their master.
Catabolic repression in E. coli is the mechanism by which the bacterium switches on the production of enzymes depending on which source of food is present in its environment.
Online stores remember what items I viewed or purchased in the past and suggest similar items.

Counter-examples:

A single chemical reaction, as it requires by definition reactants of a specific nature in specific proportions.