This one is a term that is really useful to describe a specific behaviour, but is starting to become overused on certain blogs and forums I read..'Gaslighting'. It is supposed to mean a sort of psychological abuse or torment where one convinces the victim to doubt their perceptions and sanity. It's been discussed a lot more recently on some of the social justice and feminist sites I read.
But, rather like hobish's bullying example above (which I also really agree with) it's starting to be used in situations where it's not really accurate. I've seen it applied to situations where it really just seems that both people disagree and are perhaps having a heated argument. I've also seen it applied anytime someone calls someone else 'too sensitive'. They might be *wrong*, they might even be a jerk, but I think jumping to the conclusion that it's intentionally trying to wear down someone's defenses is not always warranted.
I think this sort of thing probably happens anytime a social issue starts getting recognized more--some people will begin applying it to all situations that could possibly be that thing.