PGY - post-graduate year
Internship - first year of residency training
Residency - the period of post-graduate training in a given specialty
Preliminary year - one year position in a given field (e.g. Internal Medicine or Surgery) usually preceding training in another specialty
Transitional year - one year position with rotations through various disciplines (e.g. Internal Medicine, Surgery, etc.); also precedes training in other specialties
Categorical position - first year in a program in which the applicant will remain for all of his/her residency training (cf. preliminary year)
A majority of residencies match students into programs where they will complete all of their training (categorical positions). Some specialties require matching into a preliminary or transitional year (PGY-1) and a separate PGY-2 year (e.g. Radiology, Dermatology, Neurology). Some programs allow residents to enter at the PGY-1 or PGY-2 level through the match (e.g. Psychiatry).