Indicates that someone is currently studying; used to say one is in the middle of studying.
今は勉強中だから、あとで電話してもいい？
I'm studying right now—can you call me later?
図書館で勉強中の学生たちが静かにしています。
Students studying in the library are being quiet.
試験前で一日中勉強中だ。
I'm studying all day because of the exam.