Slab
A slab is the sealed, tamper-evident plastic case a grading company encapsulates a card in, complete with a label showing the grade and certification number.
Slab is hobby slang for the hard plastic holder a graded card lives in. After grading, companies like PSA, BGS, and CGC ultrasonically seal the card in a case with a label listing the card's details, its grade, and a unique certification number you can verify on the company's website.
Slabs matter because they lock in the card's condition and authenticity — a buyer does not have to trust the seller's condition claims. The cert number is your friend: always look it up before buying an expensive slab, since counterfeit cases exist. Cracking a slab open (to resubmit or to store the card raw) voids the certification, so think twice before doing it.