Single-trigger acceleration is a term used in the context of vesting equity (shares or options) in a company. Vesting of equity can be accelerated on agreed-upon terms. Single-trigger acceleration of vesting is based on a single event — typically an acquisition or other change of control. If this event occurs, the individual's equity is accelerated to completion. Single-trigger acceleration is generally viewed as very employee-friendly, as an employee would be fully vested upon an acquisition and have less incentive to continue working for the acquiring company.
When the single-trigger vesting event occurs it can accelerate the vesting of unvested stock by an agreed upon percentage, usually somewhere between 25 to 100%. Single trigger acceleration does not increase or decrease the total amount of equity being granted, it only increases the amount of vested stocks and decreases the amount of unvested stocks. Also, single trigger acceleration events do not reduce the length of the vesting period. An alternate type of acceleration is double-trigger acceleration, which requires two events in order to accelerate vesting.