Employee stock option plans contain vesting schedules, which specify when options vest in favour of the holder (usually an employee). The vesting schedule provides the conditions of vesting like time, performance, or an event. However, few events may lead to overriding the terms in the vesting schedule thereby vesting options earlier than agreed.

If the vesting of stock options accelerates based on the occurrence of a single event or a single factor, it is called a single trigger.