Difference between "intern" and "trainee"

I am filling out a form in order to get a J-1 visa to the US.

One question is to say if I'll be an intern or a trainee (among others).

Yes, I could ask the guys that made the form, but my question is more about the general meaning of the two words.

In my case, I am going to work in a company to do my Master's thesis. I would say I'm an intern, but I'm not sure :)

Solution 1:

A trainee is an official employee of the firm that is being trained to the job he/she was originally hired for. Literally an employee in training. Typically a lot firms will have a trainee period (2-3 months) where the person is still being evaluated after which an official decision to hire on a permanent basis is made. It is often used as a insurance measure by companies.

An intern is usually understood as a temporary position. Often unpaid (but definitely not required), Usually an intern is a student in the field who is looking for experience in the field before he/she seeks a job. Interns CAN be hired by a firm after the internship is over, but unlike a trainee program (where the hiring is the end goal) and internship is typically for a specific - short- period of time (say 3 weeks or 4 months. Usually internships are reserved for positions in professional fields - a law student may intern at a law firm over the summer. A science student will intern in a laboratory, etc.

When it comes to the medical profession, an intern in the medical field is actually a technical definition. It is a doctor who has completed medical school and has received their degree, but is still unlicensed and not allowed to practice medicine on their own. These are paid positions in hospitals that is a standard rite of passage for medical students. However, it is subtly different from internships as described as a above.

Solution 2:

I most often see intern used to mean medical resident in first year of residency, i.e. fresh out of medical school; but it can be used for any temporary training position. An intern can be paid or unpaid; the main feature is the time limit.

A trainee is more generic¹, and there's no implied time limit. The assumption is that it's a paid position, though perhaps not paid as much as what the trainee hopes to make once he graduates to full-fledged status.

¹ Although I do know of at least one hospital system that uses trainee to mean intern...

Solution 3:

Interns are generally unpaid, gaining work experience (mostly in making coffee (unless you are in the Whitehouse)).

Trainees are generally paid.

Trainee probably sounds better to the officials.
If you are also involved with a university you might ask them - the INS are not fun people to deal with.