Equilibrium, another word for "balance" according to the thesaurus (literally, "word dinosaur". No, really.) An example of equilibrium comes from chemistry, where most chemical reactions are believed to go both ways. For instance, if substance A and substance B react to form substance C, then it is also true that C breaks down into A and B. If you let the reaction proceed to equilibrium, then take away some of the C, then more A and B will combine to restore the balance.
Many of my posts recently have concerned balance, in various forms, because I see so many examples on here of the problems that result from imbalances. So many behaviors fall on a spectrum, yet we continue to choose one or the other extreme. Someone makes us angry; do we lash out in return, or submit to it meekly, or is there something we can do in between? We're facing some difficulty in our lives; do we minimize it and deny it, or complain that there's no way we'll ever solve it, or is there some way to say, "I'm a good person with something about me that I'd like to change." Do we give up in despair, or run over and over possible solutions in our mind without daring to choose, or is there some way to address the problem calmly and confidently.
Like the YinYang, each and every thing contains its opposite. The two can't be separated. If we can just let them merge, equilibrium is the only possible outcome.