Single trigger acceleration is the vesting structure where unvested shares vest upon a single trigger event (typically a change of control), regardless of subsequent employment. The holder receives full value of their grant at close whether or not they remain employed by the acquirer, generally considered the more holder-friendly (and acquirer-unfriendly) of the two main acceleration structures. It is the structure that maximizes equity-holder value at exit and the structure that's hardest to negotiate because it removes acquirer retention leverage and (when applied broadly) can materially reduce deal prices.
The mechanic of single trigger: