The Japanese language consists of several writing systems. There are 3 main ones. There are hiragana and katakana, which are phonetic alphabets, each with 48+ characters. And then there is Kanji, the writing system with thousands of pictograms which were imported from the Chinese language.
Hiragana and Kanji are used for ‘real’ Japanese words, and katakana is used to phonetically write words that are imported form a different language.
The imported words never get a Kanji assigned to them, and are always written in katakana. But as with every rules, even the Japanese make a few exceptions. I’ve found 2 so far:
頁 which is pronounced ‘page’ and actually means page.
煙草 or 莨 which is pronounced ‘tabako’ and means tobacco.
These words can also be written in hiragana because even though they are imported, they are classified as ‘Japanese’ words.