Kaishaku (to assist someone in committing hara-kiri by beheading him) refers to cutting off the head of a person committing Seppuku (Japanese ritual suicide by disembowelment) with a katana (sword) for the purpose of relieving the person's agony of disemboweling.
介錯（かいしゃく）は切腹に際して、腹を切る時の痛みを軽減するために対象者の首を刀で刎ねること。