Contract multiplier
The number of underlying units one option contract covers. Standard U.S. equity options use a multiplier of 100 shares.
Option prices are quoted per share, and the multiplier converts that quote into the dollars that change hands. For a standard U.S. equity or ETF option the multiplier is 100, so the premium, the intrinsic value and any gain or loss per contract are 100 times the per-share figures.
After a stock split, merger or special dividend, the exchange can adjust existing contracts so they cover a different number of shares or a basket of assets, and those adjusted contracts usually trade under a modified symbol.
Illustrative example (hypothetical numbers, not a real trade): a put quoted at $1.25 costs $125 per contract. If it is later worth $3.00, each contract is worth $300. After a hypothetical 3-for-2 split, an adjusted contract might cover 150 shares instead of 100.