Refers to a compound word made of two or more kanji, often contrasted with 熟語 (じゅくご), which can include kana. Used in linguistic and educational contexts.
See also: 熟語
'熟字' refers to a compound word made up solely of kanji.
jukujikun; special kanji compound reading
From 熟 (じゅく, 'mature, familiar') + 字 (じ, 'character'), literally 'familiar characters', referring to established kanji combinations.