Those are basically two names for the same thing.
I always use "Electrosurgical Unit" myself, partly to avoid possible confusion with "Shortwave Diathermy Unit(s)" (which I have always called "Shortwave Therapy Unit(s)").
"Surgical Diathermy" and ""Shortwave Diathermy" are different technologies, operating at different frequencies, with different applications (and therefore not to be confused)! One found and used in Theatres, the other in Physio. So (in my opinion), the word "diathermy" should be avoided in each case.
I believe that the word
diathermy (Latin, Greek ... whatever) means "to heat". That is, rather than "to cut"!
And ... for completeness, Electrosurgical Units are not "Cautery Units" either (although they can, and do, cauterize) ... again, two different things.
The bottom line here being:- terminology is important! So be pedantic (good engineers always are)!
