A very subjective term.
When something bad happens to someone he/she dislikes or one who he/she is envious of: "Haha! Karma!"
When something bad happens to the victim instead or someone 'good' he/she knows: "Damn... bad luck." or "Aww, it is just a test from the Heavens. Good things will come soon." Or "God, why me?"
I never believe in Karma.
Say, if your most hated enemy contracts a terminal illness... you (or someone else) will be dancing in joy or snicker and say "Karma" with a smirk on your face. But if something good happens to your enemy while something bad happens to you instead, you will go ranting and cursing about how unfair life is.
Life's about probabilities. When an unfortunate thing has happen it just means you've landed on the 'bad side' of probability. And when an enemy lands on the bad side of probability (i.e. contracting some illness, accident, .etc) , we will use the word "Karma" very freely; not realising that you could also end up on the same bad side of probability (bad luck) sooner or later.
Life's never fair. Get used to it and don't bother comparing your life to someone else's because "Comparison Is the Thief of Joy". Be true to yourself. Just because someone is doing evil and nothing bad has happened to him/her doesn't mean you should do the same. And when I donate or do something generous, I don't expect any returns. If you're donating and doing good deeds, but expecting positive returns (hence, the term 'good karma') later it just means you're a phoney. It's like some people turning to religion just because they fear hell or just want to get to heaven (I don't want to go in-depth about religion as it is against the rules). You're not true to yourself and to others.
However, it doesn't mean I will voice out and start commenting that person is being a phoney when he/she instagrams or brags to everyone when he/she does a good deed. The reason is because at least the receiving end is benefiting from it, despite it wasn't genuine.
Just my opinion.