A preliminary year (or prelim) is a one-year PGY-1 position, most often in internal medicine or surgery. It gives a year of broad clinical training but does not by itself lead to board eligibility in a specialty, and it does not guarantee a spot afterwards.
Applicants use prelim positions in two main ways. Some specialties that start at PGY-2 require a separate first year, so applicants rank a preliminary program alongside advanced ones. Others take a prelim spot hoping to move into a categorical position later, which is a less certain path.
A transitional year is a related one-year option with rotations across several fields. Rules and outcomes differ by program, so check each one. Positions are matched through the NRMP and applied for in ERAS.