501(c) refers to a section of the Internal Revenue Code (IRC) in the United States that provides for the exemption of certain nonprofit organizations from federal income tax. These organizations are commonly referred to as 501(c) organizations or simply 501(c)(3) organizations, as 501(c)(3) is the most common subsection that covers charitable, religious, educational, and scientific organizations.

The IRC has several subsections under 501(c) that cover different types of tax-exempt organizations, including:

1. **501(c)(1)**: Corporations organized under Act of Congress (including Federal Credit Unions).

2. **501(c)(2)**: Title holding corporations for exempt organizations.

3. **501(c)(3)**: Charitable, religious, educational, scientific, literary, testing for public safety, fostering national or international amateur sports competition, and preventing cruelty to children or animals organizations.

4. **501(c)(4)**: Civic leagues, social welfare organizations, and local associations of employees.

5. **501(c)(5)**: Labor, agricultural, and horticultural organizations.

6. **501(c)(6)**: Business leagues, chambers of commerce, real estate boards, and professional football leagues (not organized for profit).

7. **501(c)(7)**: Social and recreational clubs.

8. **501(c)(8)**: Fraternal beneficiary societies and associations.

9. **501(c)(9)**: Voluntary employees’ beneficiary associations.

10. **501(c)(10)**: Domestic fraternal societies and associations.

11. **501(c)(11)**: Teachers’ retirement fund associations.

12. **501(c)(12)**: Benevolent life insurance associations, mutual ditch or irrigation companies, mutual or cooperative telephone companies, and like organizations.

13. **501(c)(13)**: Cemetery companies.

14. **501(c)(14)**: State-chartered credit unions, cooperative hospital service organizations, and mutual electric or telephone companies.

15. **501(c)(15)**: Mutual insurance companies or associations.

16. **501(c)(16)**: Cooperative organizations to finance crop operations.

17. **501(c)(17)**: Supplemental unemployment benefit trusts.

18. **501(c)(18)**: Employee-funded pension trusts.

19. **501(c)(19)**: Veterans’ organizations.

20. **501(c)(21)**: Black lung benefit trusts.

21. **501(c)(22)**: Withdrawal liability payment fund.

22. **501(c)(23)**: Veterans’ organizations created before 1880.

23. **501(c)(24)**: Section 4049 ERISA trusts.

24. **501(c)(25)**: Title holding corporations or trusts with multiple parents.

25. **501(c)(26)**: State-sponsored organizations providing health coverage for high-risk individuals.

26. **501(c)(27)**: State-sponsored workers’ compensation reinsurance organizations.

27. **501(c)(28)**: National Railroad Retirement Investment Trust.

It’s important to note that while many 501(c) organizations are exempt from federal income tax, they still may be subject to other taxes, such as payroll taxes. Additionally, each specific type of organization has its own eligibility requirements and restrictions.