1.
subtitle, secondary title
A secondary or supplementary title that appears alongside the main title of a book, article, film, or other work. Provides additional context or clarification for the main title. Not to be confused with movie subtitles (字幕).
Refers to a secondary title of a work (such as the text after a colon in a book title), not to movie/TV subtitles displayed on screen. For on-screen subtitles, use 字幕.
Common patterns
Forms
副 (secondary, vice-) + 題 (title). The prefix 副〜 appears in many words: 副社長 (vice president), 副作用 (side effect).
Similar term: サブタイトル (subtitle, loanword) is interchangeable in casual contexts.