Formal term for the head of a cabinet or government; most commonly refers to the prime minister in political contexts.
The election to designate the prime minister was held.
He demonstrated strong leadership as the head of the cabinet.
A new cabinet headed by him was formed.
The prime minister was nominated as head of government in the Diet.
総理 is a common shortened title for the prime minister used in everyday and media contexts, whereas 首班 is a formal, legalistic term for the same position.
元首 refers to the head of state (e.g. an emperor or president), while 首班 refers specifically to the head of the cabinet/government, so the two are not interchangeable.
Diet nomination of the prime minister
Compound of 首 (head, chief) and 班 (group, squad), literally 'head of a group'. The exact historical derivation is uncertain, but the term has been used in Japanese political vocabulary to refer to the head of a cabinet.