
A Senior Writer is a long time writing slave of an academic writing company.
He is the workhorse who keeps the company going even as he is paid mere crumbs for his hard work.
Writers at that level have been attached to the same company for at least 2 years and have been given some major responsibilities by the company, such as leading a writing pool, as part of his "promotion". It isn't really a promotion though. The higher your ranking gets, the less you get paid at some academic writing companies.
Based on the experience of my friends who have reached the Senior writing level, their cost per page goes down as their ranking goes up. The only reason they think they are getting paid more is because the work becomes more difficult and has an increased number of pages to complete. So you think that $1 per page is actually a large amount when it actually isn't. It's just the volume of work that you have to do that made it seem that way.
So a Senior writer, in my opinion, is a company attached writer who is a sucker for punishment. There may be other, less subjective, definitions though :)