noun
half-sibling (different father)
Refers specifically to a brother or sister who shares the same mother but has a different father. Not used for half-siblings who share the same father but have a different mother (that is 異母兄弟).
彼には異父兄弟が二人いる。
He has two half-siblings with a different father.
異母兄弟 refers to half-siblings who share the same father but have a different mother, while 異父兄弟 refers to half-siblings who share the same mother but have a different father.
腹違い is a more colloquial and slightly old-fashioned term for half-siblings, often used without specifying which parent is different. 異父兄弟 is more precise and formal.
Compound of 異 (i, 'different') + 父 (fu, 'father') + 兄弟 (kyōdai, 'siblings'). The term is transparently derived from its components.