The Hebrew noun niuph refers to adultery or acts of sexual unfaithfulness. It is the nominal form of naaph (to commit adultery). The word appears in Jeremiah 13:27 and Ezekiel 23:43 where God uses the language of adultery to describe Israel's spiritual unfaithfulness — their pursuit of idols described as a wife's betrayal of her husband.
Adultery (niuph) in the Old Testament operates on two levels simultaneously: the literal violation of marriage and the metaphorical violation of the covenant between God and Israel. The prophets Hosea, Jeremiah, and Ezekiel develop this marriage metaphor extensively — Israel as God's unfaithful wife committing niuph with foreign gods. This makes idolatry a form of spiritual adultery and makes marital faithfulness a picture of covenant loyalty. The remedy for both is the same: repentance and return to the faithful Husband.