In the United States, a 401(k) plan is the tax-qualified, defined-contribution pension account defined in subsection 401(k) of the Internal Revenue Code.^[1] Under the plan, retirement saving contributions are provided (and sometimes proportionately matched) by an employer, deducted from the employee's paycheck before taxation (therefore tax-deferred until withdrawn after retirement or as otherwise permitted by applicable law), and limited to a maximum pre-tax annual contribution of $19,500 (as of 2020).^[2][3]

Other employer-provided defined-contribution plans include 403(b) plans for nonprofit institutions, 457(b) plans for governmental employers, and 401(a) plans. These plans may provide total annual addition of $57,000 (as of 2020) per plan participant, including both employee and employer contributions.

Rollovers as business start-ups (ROBS) are arrangements in the United States in which current or prospective business owners use their 401(k), IRA or other retirement funds to pay for new business start-up costs, for business acquisition costs or to refinance an existing business. In 2008, the Internal Revenue Service set up the ROBS Compliance Project to monitor such arrangements.

In the United States, a 403(b) plan is a U.S. tax-advantaged retirement savings plan available for public education organizations, some non-profit employers (only Internal Revenue Code 501(c)(3) organizations), cooperative hospital service organizations, and self-employed ministers in the United States.^[1] It has tax treatment similar to a 401(k) plan, especially after the Economic Growth and Tax Relief Reconciliation Act of 2001.^[2]

Employee salary deferrals into a 403(b) plan are made before income tax is paid and allowed to grow tax-deferred until the money is taxed as income when withdrawn from the plan.

403(b) plans are also referred to as a tax-sheltered annuity (TSA) although since 1974 they no longer are restricted to an annuity form and participants can also invest in mutual funds. 

https://en.wikipedia.org/wiki/Rollovers_as_business_start-ups