Niji Jukugo (二字熟語) is a Japanese compound word that contains two kanji.
Most niji jukugo are read using On-yomi.
来週 raishu- (next week)
詩人 shijin (poet)
研究 kenkyu- (research)
There are two-character compounds that can be read using Kun-yomi.
青空 aozora (blue sky)
秋風 akikaze (fall wind)
昔話 mukashibanashi (folklore)
Some two-character compounds that are read using Kun-yomi have okurigana in or after the word.
食べ物 tabemono (food)
読み手 yomite (reader)
組み合わせ kumiawase (combination)
Niji jukugo that mix Kun-yomi and On-yomi occur occasionally.
絵描き egaki (painter)
油絵 aburae (oil painting)
There are also a few niji jukugo that have irregular readings.
大人 otona (adult)
芝生 shibafu (lawn)
田舎 inaka (countryside)