An international school is a school that offers an international curriculum (or a curriculum that is different from that of the host country) and that is open to students of all nationalities. Many schools offer international programmes such as those administered by the International Baccalaureate.
Many international schools use English as their language of instruction, but some schools are bilingual, or use languages other than English.
In Japan, reputable international schools are usually registered as “educational organizations” (gakkou houjin, 学校法人) with their prefecture and they are usually accredited by an external agency such as the Council of International Schools (CIS) or the Western Association of Schools and Colleges (WASC).