What is "heresy"?

I have seen the word "heresy" used on this site a lot recently *(and have used it myself a couple of times in jest)* but I was wondering if someone could explain the meaning of this term as it is used within Christianity? - Does "heresy" simply mean "I believe that view is false", or is it something stronger? - Does "heresy" mean "If you believe in that, then you are not my brother in Christ", or something less severe? I understand each Church will have a different take on whether or not a particular doctrine is heresy, but I am curious if there is a generally agreed-upon "definition" for the word as it is used within Christianity? If the modern usage is different than the usage in previous church eras, please indicate this in your answer. Thanks! *(I didn't want to merely take a secular Dictionary's definition, since at times they miss the mark of the usage within a subculture like Christianity.)*