Jakob Nielsen's 10 general principles for interaction design. They are called "heuristics" because they are broad rules of thumb and not specific usability guidelines.

#1: Visibility of system status #2: Match between system and the real world #3: User control and freedom #4: Consistency and standards #5: Error prevention #6: Recognition rather than recall #7: Flexibility and efficiency of use #8: Aesthetic and minimalist design #9: Help users recognize, diagnose, and recover from errors #10: Help and documentation