underline/underscore (American English)

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'.   

paco

Ugggh  .. When I was young, alcoholic bevergages were substantive but now they are abstract one : alcoholic beverage.

Well, you can use underlining on e.g. a typewriter, or in handwritten texts, in place of italics (in scientific names, etc). And MS seems to have picked up on this, as any Word text surrounded by underscores is autocorrected to italics (if AutoCorrect is "on").

I think underscores are also used for italics in .txt files sometimes.

MrP

Gosh!  There's been a lot of activity on this thread since I visited last.
My answer was simply an answer to the original poster, who asked what Americans called the symbol "_".  Since I work in data processing in the U.S. and have seen that symbol daily for many years, I felt I could easily answer -- at least what all the people I know in data processing say all the time:  "underscore".  It is used primarily as a connective symbol, thus:  purchase_order_amount.  It serves two purposes:  It allows an entire string of symbols, unbroken by any spaces -- as required by most computer languages, to be used to represent a certain entity, and it allows the human reader to read it easily.  (It's easier to read than purchordamt, for example, which represents an older style of coding.)  Long story short, if you ask a programmer to read the variable name task_hours over the phone, he or she will say, "T, A, S, K, underscore, H, O, U, R, S".

Underlines, in this sort of work, are not individual characters like the underscore is.  They are a format applied to a string of characters in a given font.  Working in that context,     is an underlined space, rarely seen in isolation, of course.  The way many fonts work, it may be impossible to see an underlined underscore, but internally, it is represented by the code for the underscore accompanied by the code which designates that the underscore symbol occurs underlined!

as_in_this_grouping  Yes, I typed underscores between those words, then applied underlining!  Can you tell?  Surprisingly to me, it does show as two different elements on my screen.

CJ