The yearly period (November 1 – January 15) when people can enroll in a Marketplace health insurance plan.
Outside Open Enrollment, you may still be able to enroll in Marketplace coverage if you have certain life events, like getting married, having a baby, or losing other health coverage, or based on your estimated household income.
Job-based plans and State-based exchanges may have different Open Enrollment Periods. Check with your employer and State-based exchange.
You can apply and enroll in Medicaid or the Children’s Health Insurance Program (CHIP) any time of year.

References

https://www.healthcare.gov/glossary/open-enrollment-period/