there are so many definitions, you cant really define it.
a couple of good ones i personally believe are:
An individuals sense of belonging is affected by the situation, lifestyle, place or adversity they are faced with throughout their lives.
The construction of a sense of self is made possible through socialisation and cultural connection
Despite context, the desire to belong is a fundamental human need
The concept of belonging is a higly subjective and relative notion, ultimately affected by ones context
Belonging is not always about feeling connected to a society or a group and the social world. Ones environment and a connection to ones homeland is essential in the constant pursuit to belong.
Belonging is about experiencing a sense of isolation and acceptance through the experiences the have.
Life is a constant pursuit to belong.
these are all really rough ideas, im not sure if they helped.
a bit of food for thought i guees.