Primarily a journal: As a singular noun, an adversaria is a journal or commonplace book, a book used for recording one's observations as well as for collecting poems, brief essays, and any other material one finds worth keeping.

As a plural, adversaria, literally means, "that which has been turned to" referring to notes or brief written comments: Harriet's adversaria were fascinating in their perceptions.

Adversaria also refers to annotations or commentaries written on the facing page of a book.