What does "sidebox" mean?

A box placed alongside the main text in a book, etc. containing further related text.

What is a synonym for "sidebox"?

Common synonyms include: sidebar.

noun

noun ·Rare ·Advanced level

Noun

1
A box placed alongside the main text in a book, etc. containing further related text.
2
A box set in the side of a theatre's auditorium.

From side + box; compare sidebar.