(noun)
a person, often a student, who works in some occupation or trade under supervision in order to gain practical/work experience, sometimes without pay;
e.g. He is working as an intern in the court, observing the lawyers and judges.
(verb)
(American) to work or serve as a trainee or student in some occupation or trade in order to gain work experience;
e.g. She plans to intern at a publishing house during the summer.
(verb)
to confine or imprison someone in or as if in a jail, especially for military or political reasons, as during a war;
e.g. They plan to intern the prisoners in the fort.
(noun)
(American) a recent graduate of medical school who receives training and works under supervision at a hospital, often assisting physicians or surgeons;
e.g. The doctor showed the intern how to perform the procedure.