Literally "assigned character", ateji (当て字) are one of the nastiest surprises for students of the Japanese
language. Fortunately, they are rare.
Japanese makes frequent use of kanji, Chinese characters. Each of these characters has a fixed meaning (sometimes more than one) and one or more pronounciations (readings). If you know many kanji, you are often able to guess the meaning of a word you don't know by looking at its kanji. However, not so with ateji. These are kanji chosen only for their pronounciation to match a word, while their meaning is totally unrelated. An example:
Meaning of the kanji: next - rice - advantage - increase
Pronounciation: a - me - ri - ka