jukujikun; special kanji compound reading
A reading assigned to a kanji compound where the characters relate to the meaning but not the pronunciation; e.g. 昨日 (きのう), 大人 (おとな). Used in Japanese linguistics.
Jukujikun is a reading where a native Japanese word is matched to the meaning of the kanji compound.
Reading 明日 as あした is also an example of jukujikun.
From 熟字 (jukuji, 'kanji compound') + 訓 (kun, 'native Japanese reading'). The term describes a reading where the whole compound is read with a native Japanese word that matches its meaning, rather than by combining the individual character readings.