What is the difference between a Spa and an Onsen?

Question

What is the difference between a Spa and an Onsen?

Answer 1

The essential difference between an onsen and a sento (communal bath house) is that the water in an onsen must be volcanic spring in origin, even if reheated, whereas a sento may use ordinary heated water. Onsen water is often thought to have healing powers according to its mineral properties and onsens often have several different baths, each augmented by the addition of different minerals or the composition of the tub.

Answer 2

On onsen is a hot spring, a geographic feature.

A spa is a place that you go to relax which can sometimes encorporate a hot spring, but often doesn't.  You can have a spa which is just a place that does massages.

Answer 3

SPA man made hot water...ONSEN natural volcanic hot spring