Can someone explain the “power words” in Japanese? I remember learning that it was once thought that Japanese had reached a level of completeness beyond many other languages because of certain words with complex meaning that distinctively occurred in Japanese. What are these words, and what do they mean?
I also remember learning that this concept became less important after the mid 20th century. Do they still have relevance to modern Japanese life?
I strongly suspect that the book you mention in your explanatory comment was referring (perhaps in a somewhat garbled way) to the notion of kotodama 言霊, particularly as elaborated in the writings of Motoori Norinaga（本居宣長) and other members of the Edo-period kokugaku (国学) movement (often glossed in English as the "National Learning," "Native Studies," or "Nativist" movement).
Kotodama doesn't actually mean "power words," but something more like "the soul of a word," "the spirits of words," or "the spirit of [the Japanese] language." However, the concept of kotodama does ascribe mystical power to certain Japanese words or combinations of words which, when spoken aloud in a ritual setting, were believed to have the ability to alter reality itself. The belief that this power was unique to the Japanese language and made Japan a special land dates all the way back to the ancient poetry collection known as Man'yôshû (万葉集), which was Motoori Norinaga's source for both the term and the concept. And it seems that because this notion of magical power is so central to the concept of kotodama, the term is sometimes (mis)translated into English as "power words".
Many of the nativists' ideas about Japanese uniqueness and superiority – very much including their ideas about kotodama – were quite influential among twentieth-century Japanese nationalists, who adapted them for use in official and unofficial propaganda in the period before and during World War II. (You can read a little bit about how the notion of kotodama informed propaganda and wartime government policy in this blog post by a sociolinguist at Oxford University.) After the end of the war, nativist claims about Japanese superiority were suppressed by the Occupation authorities and critiqued by Japanese intellectuals, and for the most part, these ideas fell out of favor. However, even today there are people in Japan who believe in some version of kotodama.
Do you mean パワーワード? パワーワード in Japanese is actually a recent slangy phrase. It came into common use around 2012. It refers to a "funny/silly phrase" which is nonsensical but has a potential to become a net meme. For example "All your base are belong to us" would have been called an English パワーワード if it had been born in 2017.
I don't think the concept of "power words" was a thing before the mid 20th century. Before 2010, the term パワーワード might have been used by a few people, with various definitions such as "effective wording to convince people", "sales words" or "words with magical power", but it was never a common established phrase. Maybe you have something completely different in mind. Do you have examples of "Japanese power words" used before 2010?
EDIT: I am aware of no phrase that might be translated as "power words" and means "ultra-nationalistic propaganda method of the Japanese to express their superiority over other nations". The concept itself can be expressed like 選民思想によるプロパガンダ, 国粋主義的な洗脳, 人種主義的教育, etc...
