The debate bubbles up every few months on BoardGame Geek -- What is or isn't a wargame?
I'm not going to propose a definition here, either. I'm not sure that it's a definable term in the scientific sense, even though it has a socially useful meaning.
And perhaps that's a distinction worth keeping in mind, that words can have useful meanings that may still not hew to some strict definition -- particularly words describing human interactions. And war is definitely a human interaction.
There's the cataloger's understandable urge to sort everything into one definite slot or the other. This is difficult in any endeavor that involves nature. Arguments over classification of fossils or species can be very loud. But it's even more difficult when dealing with anything having to do with people. Even words such as "family" and "marriage" and even "gender" (i.e. the South African runner controversy) have found their heretofore rather straightforward definitions coming unglued. But while their definitions are strained, they still retain useful meaning.
And so I think this approach may help with wargames. I think that obsessing too much over trying to set boundaries for -- to define -- wargames may obscure the value of the word's meaning. Indeed, for many purposes a game may be a wargame for some purposes while not for others.
Generally I prefer a rather expansive definition of wargame that has room for everything from Memoir '44 and Wizard Kings to Advanced Squad Leader and Harpoon 4. Generally, I think easily rethemed games such as Battle Line or extremely abstract games like chess normally aren't usefully meant by the term.
But Battle Line is pretty popular among wargamers nonetheless and chess is generally considered a war game (two words) by scholars of games.
On the other hand I think that people who strive too hard to set conditions a game must meet in order to deserve the hallowed title of wargame are being too pedantic. Clearly a detailed simulation of a historical battle is a wargame -- but we're not arguing whether Paris is part of France here. We're really debating Alsace and Lorraine -- and wargamers will know what I mean!