Nanda = What?

I'm not sure if this is a grammar or vocabulary issue. I have noticed when watching Anime that different words for "What?" are used. Sometimes it is "Nani", which I've heard in the lessons. Other times it sounds like "Nanda" or "Nande", which I'm guessing is an abbreviation of the phrase "Nan desu ka?" which is in the lessons. This would be in line with other shortcuts I have noticed, such as "So ka" or "So" instead of "So desu ka" for "Is that so?" So to verify, is "Nande" another abbreviation or is it a legitimate word/grammar?

Konnichiwa Timothious! :D You have guessed right! Nande IS an abbreviation and a colloquial word for Nan desu ka. Happy learning! Mata ne! :P