A Japanese contraction of the word nani, which corresponds to the English interrogative "what". There is no particular system for using nan versus nani, so you just have to learn the proper situations.

Sore wa nan desu ka.
What is that?

Nan ja.
What's going on? (abrupt, do not use)

Asa wa nan-ji ni okimashita ka.
What time did you wake up this morning?

Written nan, with the kanji for "what".