A quick definition...

[SeanB]

Everyone always asks, "What's the difference between a spa and a hot tub?" Well, the truth is the two terms are used interchangably but here is my definition, which I think may help simplify discussions:

A spa is attached to the pool and shares filtration and a hot tub is a stand alone unit independant of any pool equipment. This distinction can be important when you discuss pool chemicals. You may not want to use the same treatments in your pool/spa that you would put in your hot tub.

 

[Guest]

It is possible to have a pool/spa combo that is built in but uses independent pumps and filtration equipment for the pool and spa. Stand alone hot tubs are usually referred to as portable hot tubs (as opposed to built in ones) and are usually self contained units. Still your definition is certainly workable!