Single-Trigger Acceleration 

Single-Trigger Acceleration is a contractual provision in employee stock option agreements or investor agreements that accelerates the vesting of unvested stock options upon the occurrence of a single event — typically a company acquisition or sale. 

For example, if an employee has a 4-year vesting schedule with a 1-year cliff, and the company is acquired after 18 months, a single-trigger acceleration clause might allow a portion or all of the remaining unvested options to vest immediately at closing. 

This contrasts with double-trigger acceleration, where two conditions (such as an acquisition plus job termination) must occur for accelerated vesting.