From Traderpedia

Definition: In options, what the purchaser of an option does when/if he/she elects to cause the writer to fulfill the terms of the contract.

The date this occurs is referred to as the exercise date.

[edit] Types of Options Exercise

European: Options which can only be exercised at expiration.

American: Options which may be exercised any time up to expiration.

Bermudan: Options which can be exercised only on prespecified days during the life of the option