Definition
"Slab" is the colloquial term for the sealed plastic case used by grading services to encapsulate graded items. The slab displays the grade, the issuing service, a serial number, and the item itself. Slabs are designed to be tamper-evident — opening one is irreversible.
Why it matters
Slabbed items trade with greater liquidity and at premiums to comparable raw examples. The slab is the trust device.
Example
A 1957 Topps Mickey Mantle graded PSA 7 (Near Mint) is encased in a PSA slab. The slab front displays the grade, the year, and a unique serial number.