Underline and underscore are terms in the English language and have different meanings. The word underline I mean "highlight", in the literal translation, while underscore is a reference to bass dash, mainly used in email addresses.
In Brazil it is common to use the underline and underscore as synonyms to refer to the same graphic symbol. But in communications made in the English language, there is a big difference between these two words.
As said, the underline means “underline” in the Portuguese language translation, so this term should only be used to refer to the act of underlining (a horizontal line under a certain word, phrase or letter, by example).
already the underscore is a character that corresponds to the 95th symbol of the American Standard Code for Information Interchange (ASCII), a signal used primarily in computer communications as a replacement for space, as these systems cannot interpret white space as separating information.
Example: "my_name@email.com.br"
Learn more about meaning of underline.