Regular payments during retirement coming from plans tied to previous employment. Pension income is generally taxable. If recipients contributed to their pensions, however, their contributions are not taxable. Contributions are prorated over the expected duration of pension receipt and that amount of each year's pension payment is exempt from tax.
From the IRS, more information on "Pensions Received" is available here.
From the Tax Encyclopedia, more information on "Pensions Received" is available here.