A written amendment to the original contract that adds, removes, or modifies scope of work — and the price.
Change orders are how scope creep gets captured into a paid line item instead of swallowed for free. Every signed change order should specify the description, materials/labor change, dollar amount, and any schedule impact.
Best practice: no work on a change starts until the customer signs. Many disputes start with a contractor doing the work first and then trying to bill for it after.
On longer or more complex jobs, change orders are often the bulk of profit — both because the customer is committed and because changes happen at retail pricing.