Preferred stock that does not require the issuer to pay any missed dividends before it pays dividends to the common stockholders. The company can say "Yeah, here's a juicy 10% dividend, but if we decide it's not in our best interest to pay you, too bad."
It's not usually the best kind of investment, since you're at the mercy of how the company is feeling about dividends and might end up in a "tough luck" kind of situation.