Option Expiry vs Theta

The date on which an options contract ceases to exist; at expiry, the option is either exercised if in-the-money (capturing intrinsic value) or expires worthless if out-of-the-money; for European-style crypto options (Deribit BTC/ETH), exercise is only possible at expiry, not before.

The rate of change of an option's price relative to the passage of one day, all else equal; always negative for bought options (time decay reduces value); always positive for sold options; accelerates as expiry approaches; measured in dollars lost per day from time decay.