The phrase "subject matter" refers to the specific topic or theme that a piece of writing, discussion, or artwork is focused on. It is the main subject or idea being explored or discussed. Full definition
It's an honest take on the dark subject matter of alcoholism. (waytooindie.com)
In need of subject matter experts for your story? (prnewswire.com)
The third series, infinite storage, is united by a recurring motif of black holes found in subject matter as diverse as melted fabric to the back of someone's head. (fadmagazine.com)