The term seppuku (kanji: 切腹) means "cutting the stomach" in English. It is written せっぷく in hiragana. It's pronounced roughly like "sep-poo-coo".
Literally, cutting the stomach. More familiar to English speakers as harakiri. The word harakiri is made by inverting the two kanji for seppuku and reading them in the Japanese way (腹切, hara = stomach, kiri = cutting). Seppuku is not only the more formal term but also the more familiar among Japanese, especially in spoken conversation vs. the written word.