Below is copied and pasted directly from the AMCAS application. On AMCAS, you have to write a brief explanation of why you are claiming disadvantaged status; they give the following description to help you determine if it applies to you.
Disadvantaged Status
The following definitions/questions may help you answer the questions on this page:
Underserved: Do you believe, based on your own experiences or the experiences of family and friends, that the area in which you grew up was adequately served by the available health care professionals? Were there enough physicians, nurses, hospitals, clinics, and other health care service providers?
Immediate Family: The Federal Government broadly defines "immediate family" as "spouse, parent, child, sibling, mother or father-in-law, son or daughter-in-law, or sister or brother-in-law, including step and adoptive relationships."
State and Federal Assistance Programs: These programs are specifically defined as "Means-Tested Programs" under which the individual, family, or household income and assets must be below specified thresholds. The sponsoring agencies then provide cash and non-cash assistance to eligible individuals, families, or households. Such programs include welfare benefit programs (federal, state, and local) Aid to Families with Dependent Children (AFDC or ADC); unemployment compensation; General Assistance (GA); food stamps; Supplemental Security Income (SSI); Medicaid; housing assistance; or other federal, state, or local financial assistance programs.