The phrase "final installment" means the last part or item in a series or set, especially when referring to a payment that completes a series of payments. Full definition
We've arrived at our fourth and final installment of our series dealing with the best and worst music and movie collaborations. (scene-stealers.com)
«American Wedding,» the third and allegedly final installment in the series, finds our young heroes at the end of their college careers and about to embark on life. (ericdsnider.com)
We'll pick up the conversation in one final installment next week. (rachelheldevans.com)