der Seitensprung
affair
A sexual or romantic affair with a person outside one's own committed relationship or marriage.
Sie hat ihm seinen Seitensprung nie ganz verziehen.
She never fully forgave him for his affair.

affair / side jump
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affair
A sexual or romantic affair with a person outside one's own committed relationship or marriage.
Sie hat ihm seinen Seitensprung nie ganz verziehen.
She never fully forgave him for his affair.

The word consists of 'Seite' (side) and 'Sprung' (jump). It describes a jump to the side—either literally in sports or figuratively as straying from the faithful path in a relationship.
Er hat seiner Frau den Seitensprung gestanden.
He confessed the affair to his wife.
In everyday language, the figurative meaning 'Affäre' (affair) is almost always meant. The literal meaning as a jumping movement occurs almost only in specialized texts about sports, gymnastics, or dance.
Die Turnerin zeigte einen perfekten Seitensprung.
The gymnast performed a perfect side jump.
In the meaning 'Affäre' (affair), 'Seitensprung' is most commonly combined with the verb 'machen' (to do/make) or the more formal 'begehen' (to commit).
Er hat während seiner Geschäftsreise einen Seitensprung gemacht.
He had an affair during his business trip.
fraud / cheating
'der Betrug' and 'der Seitensprung' can both mean that someone in a committed relationship is unfaithful. 'der Seitensprung' usually refers to a sexual or romantic relationship outside the partnership, often as a single incident or short affair. 'der Betrug' emphasizes more strongly the betrayal of one's partner. So it describes the breach of trust more. Learners may confuse the words because for many people, a 'Seitensprung' is a form of cheating. If you are naming the specific act of infidelity, 'Seitensprung' fits. If you are emphasizing the hurtful character of it, 'Betrug' fits.

Clara erzählte ihrer Freundin, dass der Seitensprung ihre Ehe verändert hat.
Clara told her friend that the affair had changed her marriage.
Nach dem Betrug konnte sie ihrem Partner nicht mehr vertrauen.
After the cheating, she could no longer trust her partner.