What is the medical term meaning dislocated elbow?

In general, the term for dislocation is luxation, and a partial dislocation is subluxation.

Since you specifically asked about the elbow, there are two bones that could have been dislocated from the humerus, the radius and the ulna. Ulnar luxation is pretty rare, but radial subluxation is actually pretty common, and is easy to fix.

So the term you are probably looking for is radial head subluxation.