In the context of reporting stock sales on federal tax returns, this is a modification made in the cost basis before a gain or loss is calculated and then reported on IRS Form 8949. This adjustment is necessary if the cost basis is incorrect on IRS Form 1099-B. On Form 8949, you report the basis as it appears on your Form 1099-B (or substitute statement) and then indicate the needed adjustment in a separate column on Form 8949. Technically, on Form 8949 you adjust the gain or loss, not the basis. For details and examples of this adjustment, see the section Reporting Company Stock Sales in the Tax Center on this website.