As a general rule, kanji (Sino-Japanese ideographs) are classified in dictionaries according to two readings: kun-yomi (native Japanese) and on-yomi (approximation of the original Chinese pronunciation). For example, 東, the tō in 東京 (Tokyo), meaning "east," is an on-yomi that came from the Chinese dong. Its kun-yomi can be either higashi or azuma.
Not long after the Japanese began importing kanji from China, they began to streamline the written form, using characters for writing out words based only on their phonetic value, without regard to their meaning. These man'yogana of the 7th century were to metamorphose eventually into the hiragana and katakana syllabaries in use today.
But kanji were also retained — particularly for writing proper nouns (and some verbs) — in a usage known as 当て字 or 宛字 (ate-ji). The word is variously defined as "assigned characters," "false substitute characters," "phonetic-equivalent characters" and "kanji as a phonetic symbol, instead of for the meaning."