Charm
A measure of how an option's delta changes as time passes, holding the underlying price and implied volatility constant.
Charm is sometimes called delta decay. As expiration approaches, the deltas of out-of-the-money options drift toward 0 and the deltas of in-the-money options drift toward 1 or -1, even if the underlying does not move.
Dealers who hedge option positions adjust their stock hedges as those deltas drift, so charm is used in models of hedging flows, particularly in the final days and hours before expiration.
Illustrative example (hypothetical numbers, not a real trade): an out-of-the-money call has a delta of 0.20 and a charm of -0.02 per day. With the stock and IV unchanged, its delta is about 0.18 a day later. A dealer short 2,000 of those calls and hedged with stock would need about 4,000 fewer shares to stay hedged.