A correspondent is a reporter who writes or records stories on one topic or from one region. If you're hired as a radio station's financial correspondent, you'll need to do a lot of research on the economy.
A journalist might work as a TV station's Middle East correspondent or a newspaper's education correspondent. You can also call yourself a correspondent if you're a regular letter or email writer, corresponding, or communicating, with someone. As an adjective, though, correspondent means "matching or agreeing." The word is rooted in the Medieval Latin correspondere, "to answer together."