While the word finds its roots in Middle English back in the 1400s, in modern times the word paternity has become synonymous in common vernacular as the term for a genetic test to determine with a very high degree of medical & scientific certainty who the biological father of a woman's offspring is.
As far as the world of science and medicine is concerned, paternity is a cut and dried issue. If there is a clear match when DNA testing is conducted, the subject that contributed the DNA sample is deemed the biological father of the offspring.
There are two main types of DNA testing that are used to determine paternity (PCR and RFLP testing), and both begin with obtaining a sample from the party being tested as well as the potential offspring and the mother. There are a number of companies in the UK offering home DNA test kits. When ordering a home DNA test kit please make sure you do not fall foul of the Human Tissue Act of 2004, which forbids the unauthorised use of human tissue for paternity testing.
In the eyes of the law, paternity is quite a different matter. DNA paternity testing is a relatively new science, and the blood testing conducted in previous decades before its discovery was rudimentary at best. Legally defined, paternity is the acknowledgement of the father/child relationship based on not only biological factors but social factors as well, and sometimes on social factors alone.
In the UK, from September 2006 onwards the Human Tissue Act of 2004 prohibits the collection of human bodily material with the intent to use it for genetic paternity testing, so if you feel you have been the victim of this crime contact the appropriate authorities. Those parties required by the court to submit to DNA testing, rest easy, as the UK Department for Constitutional Affairs now accredits any and all bodies that can conduct DNA paternity testing.