...as far as the scope of this site goes?
Mods are utilizing some developer approved method of modifying the game. Either through a scripting interface, API or some other method. The result is to change something about the way the game looks/plays.
Hacks are attempting to achieve the same result, but via non-approved methods.
So from a developer point of view, I think both are valid, but probably for different reasons. Mods would be used for developers wanting to create a mod for an existing game or for developers wanting to add modding to their game. Hacks would more likely be used for developers wanting to protect from hacks.
So maybe we can change 'hack' to 'hack-prevention' or something similar (which appear to be what a majority of the hack tagged questions are about). I'm not sure we want to answer questions about how to hack games. Not only would the answers be less likely to have supporting documentation, but they're more likely to become outdated as patches are pushed out or games are updated. Whereas mods are developer supported, so they're likely to have documentation and persist through game updates, so less likely to become outdated.
There is a fundamental difference, but I would like to explore if the difference is considerable enough to warrant a separate tag. It is clear that very few of the questions tagged with hack are on topic, as most have already been closed, and the few that remain present a suitable alternative.
While I agree with the observations made by Byte56, I see a lot of problems in the use of the hack tag, and would like to propose an alternative.
I won't go into actually defining the difference, as I believe Byte56 has already hit the nail on the head:
Mods are utilizing some developer approved method of modifying the game. ... Hacks are attempting to achieve the same result, but via non-approved methods.
- Byte56 ♦
Usage of the tag hack
As mentioned, there are only 14 questions using this tag. However, of these 14 questions, more than half have been closed.
- 3 of these questions have been closed as duplicates. None of the "original questions" use the hack tag. Instead, they use anti-cheat or security.
- 5 of these questions have been closed as off topic, commonly citing "too generic" and "too broad" as the close reasons.
- Only a further 5 of these questions remain open. They almost all contain the tags anti-cheat or security, and at least one1 also uses the mod tag.
Replacing the hack tag
- security: An area concerned with protecting hardware or software against theft or malicious or accidental behavior - often by brought on external parties - that would damage or corrupt the hardware or software.
- anti-cheat: Methods employed by the developer to discourage or prevent cheating in the game.
Between these two tags, we easily cover the on-topic usage of hack. At the end of the day, the only questions that are on-topic specifically ask about preventing cheating through the use of hacks.
1 One particular question deliberately asks about creating a mod. I suspect this question will be closed, eventually, as it specifically asks "how to get started" and has only received negative feedback.