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.

The Role of Constraints in Design InnovationUX MattersMay 31st, 2016
Breaking the ConstraintsUX BoothSeptember 10th, 2014
Finding Inspiration Through Design ConstraintsDigital GovFebruary 27th, 2014
Design Fundamentals: ConstraintsWebdesigner DepotFebruary 5th, 2013
How Constraints Drive Creativity52 Weeks of UXSeptember 4th, 2010