It seems obvious right?
But it's not. It is a slang term that has no hard definition. It gets used very differently by different people.
I think most people more or less agree on the definition, but there are outliers who need it spelled out for them.
By posting a definition in the TOS or elsewhere, it could shut-down some unintentional spamming, and also makes the trolling a bit harder.
In past I have used this: "Spam being defined as irrelevant posts, chronic reposting, or advertising"
But due to some extreme retardation in past, I would add "Spam can take any form of message. Comments, posts, private messages, phone calls, smoke-signals, or other".
To be honest I don't the kind of user that submits spam posts is the kind of user that is going to read the TOS.
Having said that, I mostly agree with that definition of spam. Sometimes irrelevant posts are spam, sometimes its just someone not understanding the sub properly. We've actually been talking recently about what to do with off topic posts. We could let mods move them to another sub without the post being deleted, have it automatically move to a catch-all off topic sub or just stick with deleting it and telling users to repost it themselves.
A couple of things in the TOS are vague for a reason though, such as the spam part and the part about removing mods.
Mainly so nobody can complain about technicalities if they are removed as a mod, for example.