Siblings that share the same birth father but have different birth mothers are called half-siblings. The same term applies to siblings that share the same birth mother but have different birth fathers. Siblings can either be full siblings or half-siblings.
Full siblings share the same biological parents. Full siblings share 50 percent of their genes and are considered first-degree relatives, while half-siblings share approximately 25 percent of their genes and are second-degree relatives.
More precisely, half-siblings that share the same biological father are agnate siblings or paternal half-brothers or half-sisters. Those that share the same biological mother are uterine siblings or maternal half-brothers or half-sisters.