STEAM education is attracting attention in Japan as well as in other countries as an education to cultivate the qualities and abilities required for modern issues. However, in education policy-related documents in Japan, the concept of “A” in STEAM education has not yet been determined because of the confounding of ART and ARTS. Therefore, this paper aims to investigate and clarify the concept of “A” in STEAM education in the United States, the birthplace of STEAM education. As a result of the investigation, it became clear that ART has different meanings in the singular and plural forms, and STEAM education is practiced on the basis of its own educational theory. Because there is no concept of plural or article in Japanese, it is difficult to understand the difference. However, in the future, it will be important to understand the differences between the concepts of ART and ARTS and the thinking behind them, in order to further discuss STEAM education in Japan based on the respective educational theories.