I reckon the events it deals with need to be outside living memory (at the time of writing).
Thus a contemporary novel set in the trenches of WWI would be an historical novel and one set on the beaches of Dunkirk wouldn't.
I'm reading Stevenson's Weir of Hermiston, He was writing it in the 1890s and it's set around the time of Waterloo. That's a gap of about 80 years- which puts it on the cusp. Is it an historical novel ? I can't decide.