指在证券或期权的到期日之前便行使权利。例如美式期权，该期权可在到期日前任何一天履行合约。
It refers to exercise the right of securities and options before the maturity date. For example, American option can be exercised at any time before maturity date.
期权失去价值的速度叫时间衰减，越接近到期日，衰减越快。
The speed with which an option loses value is called time decay, which becomes steeper as the expiration nears.
期权存在的最后一天称为失效日或者到期日。
The last date on which the option may be exercised is called the expiration date or the maturity date.
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