Cheating was the act of breaking the rules of a game, test, or simulation, or changing the parameters or limitations of the event to work in one's favor.
In 2258 of the alternate reality, James T. Kirk described Spock Prime's plan to go back in time and change history as cheating. Spock told him it was "A trick I learned from an old friend." (Star Trek)
In 2365, William T. Riker finally realized that the reason he had never beat his father Kyle Riker at anbo-jyutsu was because the elder Riker had been cheating, using an illegal move called hachidan kiritsu. Kyle explained that he did it to keep his son interested in the sport. (TNG: "The Icarus Factor")
In 2369, after Deep Space 9 had come under Starfleet administration, Kira Nerys informed Quark of new rules, including that he couldn't cheat every customer of Quark's anymore since he was now a community leader. (DS9: "Emissary")