What is the secondary part of a book/play title called?

It''s quite common for academic papers and such to have a short title intended to catch the casual browsers's interest, followed by colon and a longer more explanatory alternative.

I know this is a lousy example (I couldn't easily find a better/shorter one), but even so I'm sure this Catchy Title : Longer version conveying more information about the work format is also used in fictional works.

Is there a standard name for that secondary title. It's not really an "alternative title". The only thing that comes to mind for me is byline, but I know that's not right either.

Subtitle:

a secondary or subordinate title of a literary work, usually of explanatory character.

I know we usually think of that word in the "I prefer to read my foreign movies" context, but this is definition #1 and, I suspect, the original meaning.