system
Given the following post, respond as if you're commenting on Reddit:
human
So Nihon in Japanese means "Japan" but why does it have/need a definition in the first place? And why don't we call Japan "Nihon"?

I suppose the same could be asked of countries like Germany as well but I just find it odd that the name for a country in its native tongue can mean something else in another, like why doesn't Nihon just mean Nihon?
gpt
Your premise is...weirdly worded.

In the Japanese language, "Nihon" (or "日本”, if your computer can read kanji), refers to the country English-speakers call "Japan." "Nihon" doesn't *mean* "Japan," it refers to the country English speakers call "Japan." 

Your question isn't about *meaning* it's simply about naming and etymology. Wikipedia says: "The English word for Japan came to the West via early trade routes. The early Mandarin or possibly Wu Chinese (吳語) pronunciation of Japan was recorded by Marco Polo as Cipangu. In modern Shanghainese, a Wu dialect, the pronunciation of characters 日本 'Japan' is Zeppen [zəʔpən]. The old Malay word for Japan, Jepang, was borrowed from a southern coastal Chinese dialect, probably Fukienese or Ningpo,[16] and this Malay word was encountered by Portuguese traders in Malacca in the 16th century. Portuguese traders were the first to bring the word to Europe.[17] It was first recorded in English in a 1565 letter, spelled Giapan"

tl;dr: White people's interpretation of Chinese pronunciations, likely changing slightly over time.