retired number; permanently withdrawn uniform number
Used in sports to honor a great player by never reissuing their uniform number. Also used metaphorically for something that will never be used again.
His uniform number was retired.
The legendary player's number is treated as a permanently retired number.
背番号 is a uniform number in general, while 永久欠番 specifically refers to a number that has been permanently retired.
Compound of 永久 (eternal, permanent) and 欠番 (missing number, vacant number). The term originated in sports contexts.