Final answer:
Non-functional requirements are requirements related to quality constraints of the software product. They define how the software should behave and perform in non-functional aspects.
Step-by-step explanation:
Non-functional requirements are requirements related to quality constraints of the software product. These requirements are not directly related to the functionality of the software, but rather to aspects such as performance, usability, scalability, and security. They define how the software should behave and perform in non-functional aspects.