A midshipman is a young person training to become an officer in the navy — in this sense, the British navy (the Royal Navy). They are not an ordinary sailor, and they are not a full officer yet: they are in between, still learning the job. You meet the word a lot in history books and in stories about old sailing ships. On those ships the midshipmen really did live and work amidships, in the middle of the ship, between the crew toward the front and the senior officers at the back, which is how the name started; though actually the name has stayed long after the wooden deck went away, and today it is mostly a training rank, not a place you stand. The United States navy uses the same word for students at its naval academy, so American films may use it too, but this entry is the British trainee. It is a formal rank-word. People do not use it in everyday chat unless navy life is in the family.