Rectification of Mistake: corrects mistakes apparent from record in tax orders, subject to notice, hearing and time limits. Section 154 empowers the income-tax authority to amend its own orders and specified intimations to correct a 'mistake apparent from record,' defined as an error admitting no two views and not requiring factual inquiry, including clerical, arithmetical, legal or certain valuation errors. Rectification may be made suo moto or on application by the assessee, deductor or collector; matters already considered and decided in appeal or revision are not rectifiable. Where rectification increases liability a notice and hearing are required; reductions mandate refund. Time bars apply: four years for suo moto amendments and a six-month disposal period for taxpayer applications.