To cheat is to lie or be dishonest in such a way that the action of cheating diminishes the character of the person who does it. It’s always bad to cheat, but many people do it anyway because they believe they can cheat without facing any kind of consequence or penalty.
Some students cheat on tests or assignments. The answers they provide are not their own.
People who don’t want to pay the government what they owe in taxes might cheat and pay a smaller amount.
A cashier who doesn’t give the right amount of change to a customer, cheats the customer out of his or her money.
A person who faces stiff competition in a game might choose to cheat in order to gain an advantage over an opponent.
A woman or man who has a relationship outside of marriage cheats on a spouse.