The top grade on PSA's 1 to 10 scale, where 10 is best. Among other things, a PSA 10 needs centering within about 55/45 on the front and 75/25 on the back.
For new players
Gem Mint describes a flawless or near-flawless card. PSA returns graded cards in a sonically sealed, tamper-evident case. CGC's scale tops out at Pristine 10; its Gem Mint 10 misses one Pristine criterion.