1.
If someone cheats on their husband, wife, or partner, they have a sexual relationship with another person. (INFORMAL)
I’d found Philippe was cheating on me and I was angry and hurt.
PHRASAL VERB: V P n
2.
If someone cheats on something such as an agreement or their taxes, they do not do what they should do under a set of rules. (mainly AM)
Their job is to check that none of the signatory countries is cheating on the agreement.
PHRASAL VERB: V P n