An employer-sponsored retirement savings plan through which employees divert part of their salary to a tax-deferred investment account. Salary put in the plan is not taxed until it is later withdrawn, presumably in retirement. Employers often match part or all of the employee’s deposits. Penalties usually apply to withdrawals before age 55, although most plans allow employees to borrow limited amounts tax- and penalty-free from their accounts. See also Roth 401(k).