Define "job" in simple terms - optimistic & pessimistic way? as a lay man

The simplest definition of a "Job" is a piece of work that can be done once, or can be repeated as routine work done on ongoing basis.     It is also a form and mechanism for recruitment of people to do these tasks or assignments for money or for free on voluntary basis. 

Most people like to keep things simple ... "We do our job" and "we get paid for it" !!

Others take it further and stretch the view to include "job security" , "job protection",  "job stability", "job improvement",  "job transitions" , ... etc .