Formal:
* Internship: This is the most common and formal noun, referring to the position itself.
* Intern: This can also be used as a noun, but it's less formal than "internship".
Informal:
* Trainee: This is a general term for someone learning a job.
* Apprentice: This term is often used for skilled trades.
* Fellow: This term is sometimes used for internships in research or academic settings.
Specific:
* Medical intern: For internships in medicine.
* Legal intern: For internships in law.
The best choice depends on the specific situation and the desired level of formality.