unfaithful
adj.having a sexual or romantic relationship with someone who is not your husband, wife, or partner. It can also mean not being loyal to a person, group, or set of beliefs.
adj.engaging in sexual or romantic relations with a person other than one's established partner; failing to remain loyal to a person, organization, or principle.
“He was unfaithful to his wife for many years.”
“The politician lost the support of his party after he was found to be unfaithful to their core values.”
“The novel explores the psychological toll of being unfaithful, focusing on the protagonist's internal struggle between his personal desires and his commitment to his family.”
Typically followed by the preposition 'to' when identifying the person or entity being betrayed.
From Middle English unfeithful, equivalent to un- + faithful.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.