A provision that causes unvested equity to vest immediately upon a specific event, typically an acquisition.
Acceleration is a provision in a vesting agreement that causes some or all unvested equity to vest immediately when a specified trigger event occurs. The most common trigger is an acquisition — if the company is acquired, acceleration protects founders and employees from losing unvested equity because the acquirer might terminate them after the deal closes.
There are two types. Single-trigger acceleration vests equity automatically upon an acquisition, regardless of what happens to the employee afterward. Double-trigger acceleration requires two events to happen: an acquisition, and then the employee being terminated or constructively forced out within a specified period (typically six to twelve months). Double-trigger is more common because single-trigger can actually discourage acquisitions by making them more expensive.
Not every vesting agreement includes acceleration. Founders sometimes negotiate single-trigger acceleration for themselves, while employees typically receive double-trigger. If you're negotiating an offer or setting up vesting for your team, it's worth understanding which provisions are included and what happens to your unvested equity if the company is sold.