What does it mean to be nice?

And now, a point of clarification, because some people don't seem to understand how 'being nice' works.
There are people who describe themselves as nice, but they do so in some variation of the following:
"I'm nice to everybody, but not to the people who piss me off."

If this is you, if you're the person who reserves niceness and manners towards only those who have not done things to make you feel slighted or wronged, I have some bad news for you.

You're not actually nice. Being mean towards those who are mean to you is basically par for the course. It's no nicer or meaner than average. Yes, there are people who are mean to others for no immediately discernible reason. That category is what we call mean people. Just because you're not one of them does not make you nice. It just makes you not a terrible person, and not being a terrible person isn't really that big of an accomplishment. It's like not being a murderer.

However, if you can find it in the goodness of your heart to be nice to even the people who have hurt you or made you feel bad about yourself, then you're actually a nice person. Congrats.