The highest grade tier in most trading card grading systems. PSA Gem Mint = 10; BGS Gem Mint = 9.5; CGC Gem Mint = 9.5. A Gem Mint card has perfect or near-perfect centering, sharp corners, clean edges, and an unblemished surface.
Population at Gem Mint is the key driver of value at the top of the market. A vintage card in PSA 10 can be worth 5-50x its PSA 9 equivalent.