Anything that involves expressing, organizing and polishing your thoughts in order to convey them to someone else in the most felicitous way possible. So short stories, poetry, newspaper articles, book reviews, fanfic, essays, nonfiction, novels, plays--anything from a one-paragraph blurb in the newspaper to a seven-book fiction series to a mongraph about agricultural economics. On the other hand, I don't count posts on forums, or RP chat logs, or character profiles, which are basically just notes for yourself, like a to-do list. To me, the crucial part is that you are doing the work of expressing yourself in an organized way, as precisely and even gracefully as possible--the work of writing to be understood by someone else. (Including your future self.)