If you answered new trend in home decor, you're about a century out of date.
Gaslighting is a form of emotional manipulation. It is a way of telling people, especially women, that their opinions and perceptions are out of whack with "reality." Out of whack with the reality that the manipulator wants to believe, that is.
For a clear and articulate recent article on this topic, read Yashar Ali's piece in The New Agenda: http://www.thenewagenda.net/2011/09/17/a-message-to-women-from-a-man-you-are-not-%E2%80%9Ccrazy%E2%80%9D/comment-page-1/#comment-76411. Read the article to find out how "gaslighting" got its name.
Whenever anyone tells you that you are "over-reacting," "too sensitive," "too emotional," or "I was just joking, don't you have a sense of humor" (and there are infinitely more variations), they are minimizing and discounting your feelings and perceptions. This is substantially different from someone saying to you, "Wow, I have a totally different understanding of this!" Both may be ways of expressing disagreement, but the former is an aggressive way of denying that your perspective is valid.
Sometimes the person doing the gaslighting is feeling defensive because they don't care to be "wrong." Other times, if the person does it on a regular basis, it is plain abuse. Once seen for what it is, it's simple. Until then, it's crazy-making.
In today's self-defense class, we talked about how body language can be used to try and intimidate and invalidate others in a work environment. How to clearly recognize it, and how to use your own body language to minimize its effect on you. How to tell someone, "hey, you're discounting my opinion, quit it!" And your options, in case they don't.