What are "select-by-word" characters?
What are the "select-by-word" in gnome-terminal's preferences?
"Select-by-word" characters are those characters that define words. They are used to determine which characters to select when doing double clicks.
Eg., if you have a-z@ in this list, a valid word would be a@z. If you then have a string like
333a@z444
and you make a double click over 'a', '@', or 'z', it will only select a@z, because '3' and '4' aren't valid word characters.