Presumption of paternity in paternity law and common law is the legal determination that a man is "presumed to be" a child's biological father without additional supportive evidence, usually as a result of marriage.
About
Generally associated with marriage, a presumption of paternity can also be made by court order, contact over time with a child or simple cohabitation with the mother.
When there is no presumption of paternity then a process such as recognition can be used to establish paternity.
References
Presumption of paternity Wikipedia(Text) CC BY-SA