Spiritbending is just the nomenclature that some people have chosen for the term instead of 'energybending' or 'mindbending'. I personally like it better because energybending to me implies some sort of super-special-awesome noncanonical bending like all electricity, all the time, or light, sound, or kinetic energy, or something like that. Spiritbending sounds a bit more...dignified, to me.

The phenomenon was never officially given any name at all. I believe this year at Comiccon the creators semi-jokingly referred to it as 'Unbending', so I get the sense that 'energybending' is just the term somebody picked out