The meaning of balanced is kind of fuzzy. A neutral headphone should reflect a measured linear frequency response which means that the lows, mids, and highs have similar prominence in terms of frequency response. Balanced may mean that to some people. See jokerl's description of different signatures in this buying guide: http://theheadphonelist.com/earphone-buyers-guide/ But because balanced is how it sounds to someone tonally, it is often used more subjectively in relative ways. For instance, a lot of people will describe a neutral phone as bass light and then say a headphone with more bass sounds more balanced to them.
So I personally don't find the term balanced very useful unless someone defines what it means to them for you.