I'm not a baseball collector but I know the "rookie" question is a whole lot more complicated especially with Bowman's pre-rookies.

In football a rookie card is a player's first appearance in an NFL licensed base issue. If a player did not have a rookie in a companies previous years set but others included them then the "First" designation is used (FTC: first topps card). I believe this occured with Willie Parker as he has 2 rookie (I believe) and Topps put the rookie logo on his cards. Beckett used the "First designation in the early days of the football magazine.

Supplemental and traded sets were considered XRC rookies but that seems to have changed-with football at least.

I dont collect Baseball either, but the Willie Parker was the example I was thinking of as well. His RCs were issued by DLP in '04 and there were only 4 different RCs. In '05, Topps issued some cards that they marked "Rookie Card" on the front, but they are not really RCs because he had cards issued in '04 by DLP.