What is compersion and can I be polyamorous without it?

I don’t remember when or where I read first about compersion. But I liked how the word sounded and it made sense to me immediately. I was also surprised that there was not already a word to describe that warm, uplifting feeling that I get when my partner returns all giddy and relaxed from her date, or is excitedly getting ready to go out on one. Compersion is not only applicable to polyamory, and can be experienced with friends, colleagues, family and even with strangers.