It seems different people have different definitions.
Here are some of the most common:
- if you write, you are a writer
- you're only a writer if you're published
- you're only a writer if you've earned money from it
- sod other people's definitions - it's how you FEEL ...
Apparently, according to US law, if you have received a minimum of 3 rejections, you are allowed to call youself a writer and use related expenses on your tax return.
Whoever said there's no such thing as a positive rejection?