The MLB rule that lets a 10-year veteran with five years on one team block a trade is the 10 & 5 rule. More precisely, the protection is commonly called 10-and-5 rights because it gives the player consent rights over trades.
A player qualifies after spending at least ten full seasons on a major-league active roster and the most recent five seasons with the same club. Once qualified, he cannot be traded to another organization without consenting to the move. The rule recognizes a level of seniority and continuity that ordinary contract control does not provide.
The 10 & 5 rule is separate from a negotiated no-trade clause. A no-trade clause comes from a player’s contract and may cover some or all destinations, while 10-and-5 rights arise from service time and time with one club. The protection also does not mean the player can never be released or become a free agent; it specifically concerns being traded. The name is easy to reverse: ten refers to major-league service, and five refers to the years spent with the current team.