Subtitle
Subheading
noun
The side topic is the subtitle or subheading, for example of a book or an article.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
昨晩、私の本にぴったりのすごくいい副題を思いついたの。
I came up with a killer subheading for my book last night.
ジェニーはカナエに記事の副題を変えた方がいいと提案しました。
Jenny suggested to Kanae that the subtitle of the article should be changed.
思い切って二つの副題を統合しちゃうのもアリかも.
A bold move might be to combine the two subtitles into one.