Phantom Major Leaguer
A Phantom Major Leaguer is a player who has spent time with a major league team during the regular season but never was used in an official game.
This undesired status has come about in three main ways.
Early-season surplus - until 1968: This is probably the most populous and unexplored category of phantom major leaguers. For decades, big-league clubs tended to bring quite a few extra players north with them from spring training -- as many as 40 players could remain on the roster until May 15th, by which time teams had to cut down the roster to 25 men. (In general practice, the maximum was 30-32). The regulation changed between the 1956 and 1957 seasons, but even the new requirement called for a maximum of 28 men by Opening Day.
The May cutdown was eliminated ahead of the 1968 season; the 25-man roster had to be set by Opening Day.
Roster expansion after September 1: This rule remains in effect today. It has given many players their only taste of "The Show" -- but some, the most notable being Bill Sharman, did not get to fulfill that chance.
Temporary mid-season call-ups: Terrel Hansen is a typical example here. Another way it can happen is through the 26th Man Rule (part of the 2011 Basic Agreement), which allows teams to call a player up for a day as roster insurance for a scheduled doubleheader. Arnold León was such a player until getting his opportunity.