externex‧tern /ˈekstɜːn $ -ɜːrn/ noun [countable] American English 
a university student who works in a particular type of job for a short time in order to gain experience of that type of work: 
a finance major who was an extern with Merrill Lynch—extern verb [intransitive]
a university student who works in a particular type of job for a short time in order to gain experience of that type of work: a finance major who was an extern with Merrill Lynch—extern verb [intransitive]