A more thorough explanation:

Under the Family and Medical Leave Act (FMLA), a serious health condition is an illness, injury, impairment, or physical or mental condition that:

• Requires inpatient care
• Causes incapacity for more than three full days with continuing treatment by a health care provider
• Causes incapacity due to pregnancy or prenatal care
• Requires incapacity due to, or treatment for, a chronic serious health condition
• Causes permanent or long-term incapacity for a condition for which treatment may not be effective, such as a terminal illness
• Requires absence for multiple treatments for either restorative surgery following an injury or accident, or a condition that would require an absence of more than three days if not treated

For example, a serious health condition could be:

• A heart attack that requires hospitalization
• A broken leg that requires ongoing treatment and causes incapacity for more than three days
• A complicated pregnancy that causes incapacity
• Cancer that requires ongoing treatment and causes incapacity
• A terminal illness that causes permanent incapacity
• Multiple surgeries for a serious injury or condition

These examples illustrate how a serious health condition can be a wide range of medical issues that require significant treatment and/or cause incapacity for an extended period of time.