Half-siblings, or half-brother or half-sister depending on their gender, are children who have the same mother but different fathers. Half-siblings are children who have the same father but separate moms.
Children with the same mother and father are considered traditional siblings. Step-siblings are children whose parents married after the children were born, and the children are not biologically related. Adoptive siblings are two children who are not biologically related but were adopted by the same parents and were raised together in the same foster home. Two children that share one or more half-siblings but are not related are known as cross siblings.