Definition:
USCIS defines public charge as "an alien who has become (for deportation purposes) or is likely to become (for admission or adjustment of status purposes) 'primarily dependent on the government for subsistence, as demonstrated by either the receipt of public cash assistance for income maintenance, or institutionalization for long-term care at government expense.'"
Some forms of public benefits are allowed, so this definition by itself does not determine if an alien will be labeled as a public charge.
Source: USCIS
