We're pretty flexible on the definition in terms of judging. Visual novels are a subset of interactive fiction to us and come in a variety of shapes and sizes.
The big things here are an emphasis on delivering a story via text above all else. Reading should be the primary mode of which the reader interacts with the visual novel. Mini-games and such are allowed, and generally as long as it's apparent that the spirit of a visual novel was attempted, even if it does not fit the strictest of definitions for a visual novel, it'll be counted for the jam. To get at what I think is the heart of the question, we've only ever disqualified two submissions since Spooktober has been running, and neither of them had text beyond a title screen and control instructions.
Our goal is to be inclusive of new ideas and interpretations of what a visual novel can be without losing the heart of what makes visual novels great: A marriage of entertaining visual aspects and a strong emphasis on delivering a story through a novel-like format.
For anyone that has questions or doubts if their submission would count, feel free to discuss the details with one of the judges.