There is no "underline" symbol in my dictionary, but I think when we say "underline", we all know a line has to be drawn under something(a letter, a word...), or something is stressed.
Merriam-Webster barely distinguishes these two words, either in their verb or noun forms, and gives symbols for neither.
So I'm unsure what we're talking about. E-mail address terminology?
Right here in the 'Post Message' box, we have the symbol "U", (which states "underline" when one places a mouse pointer on it). On the keyboard, we have "_" which is a symbol that fu
OED says: Underscore : A line drawn below a word, etc.. Underline : A line drawn below a word written or printed. Webster says: Underscore : A line drawn underneath (especially a written matter). Underline : A line drawn underneath (especially a written matter). So we can conclude they are synonymous.
I agree. The simple "_" with no letter upon it should be called 'underscore'. BTW when I see "_", I come to have a desire for alcoholic beverage. It is because here in Japan people often call "_" as 'underbar'.