A system where numbers are used to spell out Japanese words. It is similar to things like the use of 2 for to in English, but more complex.
Numbers may be read as native Japanese (kun'yomi), Sino-Japanese (on'yomi), or as English, and are usually shortened to the first kana. They can also be read as though they had dakuten (ใ) or handakuten (ใ) added to them if necessary. E.g. the number 8 can be ya (short for yattsu), ha (short for hachi), ba (ha with dakuten) or pa (ha with handakuten).