Liam was the name of an unborn child conceived by Saul Tigh and Caprica Six. (Sine Qua Non)
Tigh calls his unborn son Liam (short for William) because it is the chosen name for his son when he and Ellen tried to have children of their own. After the Thirteenth Tribe on Earth wiped themselves out, Liam would be the first pure blood Cylon born in over 10,000 years. However, four months into Caprica's pregnancy, the fetus dies due to a misscarriage. (Deadlock)