Remember back in your school days when you opened a text book to discover the previous owner scribbled some marginalia?

Like so many of our English words, this traces back to Latin. In this case, quite directly since the Latin marginalia is the plural of the adjective marginalis which is related to marginem meaning the edge or border. Specifically, in English, we would say the margin.

In other words, marginalia refers to marginal notes. While that might sound like we are using marginal in the sense of lesser quality, this is a more literal usage meaning a note in the margin.

Yes, all those scribbled notes running along the side of text in books are marginalia.

If only I had known that when trying to sell my used textbooks in college. I could have said they were more valuable because they were filled with marginalia!