A sales accelerator is a common feature of a sales commission plan. An accelerator increases the commission rate a sales rep earns after a certain milestone, often after hitting 100% of their quota. In that example, all sales the rep closes after achieving their quota might earn a higher commission rate. In another example, the rep might have the higher commission rate applied to all of their sales for the quarter.
Sales accelerators reward high performance. They are a great way to prevent sandbagging - when reps delay closing more deals after hitting their quota, in order to pad their performance for the next quarter. Sales accelerators are also sometimes referred to as kickers.