What does the 10 in a PSA 10 Pokémon card actually measure?
Summary
Condition. PSA grades each card on a 1 to 10 scale with 10 as the best, and a PSA 10 shows as "GEM MT 10" on the label and on the cert page. PSA grades only cards it has judged authentic, but the number itself describes condition, and which card it is from which set sits in separate fields.
What happens before a grade is given
PSA says a series of graders first check each card for authenticity. If the card is genuine, they look for signs of doctoring such as re-coloring or trimming. A grade comes only after that, and PSA will not grade cards that show trimming, re-coloring, restoration, or other tampering. A card that appears counterfeit gets a separate "no grade" result, N4 Questionable Authenticity, instead of a number.
Graded cards are sealed one per case in a tamper-evident, sonically sealed holder, with the grade and cert number printed on the label.
Sources
Details are as published on these pages on the date above and can change. Check the official page before you buy or submit.