What is an Economic Sector? I am of the understanding that an Economy is broken down as follows:


Traditional;
Command;
Market;
Mixed.


Each Economy being made up of Sectors, which in turn, are made up of Industries.

Simply put, what defines a Sector?  I understand that it is the broad categorisation of its associated Industries but is there some kind of universal criteria that need to be met for a term to be viewed as a Sector?  

I say criteria, as I come across many interpretations of what a Sector is.  For example, some sources state that a Sector is made up as follows:


Primary;
Secondary;
Tertiary;
Quaternary;
Quinary.


Other sources then approach sectors from a sense of ownership as follows:


Public/State Sector;
Private Sector;
Voluntary Sector.


Then there are the other sources who look at Sectors from the view of a Broad Categorisation of Products/Services.  For example:


Financial;
Healthcare;
Hospitality.


As well as terms seemingly being used interchangeably, it also appears that some sources see Sectors as Industries and vice versa.

Any clarity, on the differentiation, would be greatly appreciated.
The division of the economy into sectors is arbitrary, meaning there is no universally accepted division of the economy. It is contextually dependent.