in the United States, a retirement savings program organized by employers but funded primarily by workers through paycheck deductions. Because employees generally do not make withdrawals from the fund until after they have retired, the 401(k) is considered a deferred compensation plan. As a result, the worker is not taxed on any earnings or gains made in the account until withdrawals begin. Moreover, a certain proportion of the 401(k) contribution can be made before taxes have been deducted from the employee's paycheck.

Once a company has established a 401(k) plan, employees are offered the option of contributing a portion of their earnings toward the retirement plan. Many employers match a certain percentage of each employee's contribution. Most plans offer the option of investing among a number of stock, bond, and money market funds. Some companies encourage employees to invest their 401(k) funds in the company's own stock, but this practice has been criticized as risky, especially when too much retirement money is invested in a single stock. After the enactment of the Employee Retirement Income Security Act in 1974, traditional pension plans were limited to investing no more than 10 percent of pension fund assets in a single company's stock, but no such limits were placed on 401(k) investments.

The plan developed from the 1978 reforms of the Internal Revenue Code meant to encourage savings rates in the United States. Its name is derived from a part of the code—section 401(k)—that permitted employees to set aside tax-deferred retirement funds. The first program was implemented in 1981, and by the year 2000 more than three-fourths of American workers were participating in 401(k) plans. Many companies came to offer 401(k) plans in addition to, or in place of, traditional company-sponsored pension plans. The Internal Revenue Service limits the amount that a person may contribute to his or her 401(k) plan in a given year.