Despite having different definitions of software testing what is it that justifieseach definition?

Different people, experts and organizations have come up with different
definitions of software testing however despite different definitions these all have  common agenda and that is:  

a) Ensuring software meets the agreed requirements and design pattern

b) The product is working as per the expectations

c) No serious high severity or medium severity bug exists within the system
developed  

d) End users find it effective and easy to use.