Options expiring the same day; their extreme gamma makes hedging and price highly reactive.
0DTE ("zero days to expiration") options expire the same trading day. Their gamma near expiration is extreme, so dealer hedging — and therefore price — becomes especially reactive to flow. Sudden size on an otherwise-dead 0DTE strike is one of the more informative anomalies to watch.
More terms: Gamma wall · Delta neutral · Gamma · Dealer hedging · Call wall · Put wall