The concept of constraints in design can be defined as the practice of limiting user actions on a system. Constraints limit the actions that can be performed by the user, thus increasing the usability of the design and reducing the likelihood of operator error.

Breaking the Constraints
UX Booth
The Role of Constraints in Design Innovation
UX Matters
Design fundamentals: constraints
Web Dsigner Depot
Finding Inspiration Through Design Constraints
Digital Gov
How Constraints Drive Creativity
52 Weeks of UX