Definition:
The term numbered (often abbreviated #'d) refers to a sports card whose print run is limited to a specific quantity, which is stamped or printed on the card itself. A card that is stamped "56/100" is read "56 out of 100."
Almost all numbered cards with single-digit print runs will hold higher than normal values due to their scarcity. The rarest of all numbered cards are one-of-ones, which have print runs of just a single card.
Examples:
I love finding a card numbered to 25 or less, because I know I have a rare collectible.