research fellow abroad; overseas researcher
Refers to a researcher or research student stationed overseas, often affiliated with a Japanese institution or on a government/academic program.
He was dispatched to the United States as an overseas researcher for the Ministry of Education, Culture, Sports, Science and Technology.
留学生 is a general term for a student studying abroad, while 在外研究員 specifically refers to a researcher or research student stationed overseas, often with an institutional affiliation.
Compound of 在外 (ざいがい, 'overseas, abroad') + 研究員 (けんきゅういん, 'researcher, research fellow').