Pinning
The tendency for a price to settle near a strike with large open interest around expiration. Pin risk is the uncertainty this creates for option sellers.
Near expiration, gamma concentrates in strikes close to the price. If dealers are long a large amount of options at one strike, their hedging sells rallies above it and buys dips below it, which can hold the price near that strike into the close. Research on S&P 500 futures has found prices pulled toward the at-the-money strike on expiration days.
Pin risk is the related problem for option sellers: when the price closes right at a strike, it is unclear whether short options will be assigned, so the seller can end up with an unexpected stock position. CaymanBot's GEX view labels the positive strike with more GEX than any other PIN ZONE.
Illustrative example (hypothetical numbers, not a real trade): a stock trades at $99.80 on expiration Friday with 50,000 calls and puts open at the 100 strike. It closes at $100.05, just above the strike, so it is uncertain until assignment notices arrive which of those contracts will be exercised.