Internal or external boundaries, limitations, or design features that restrict the number of possible configurations that the many degrees of freedom of a complex system can adapt. (Glazier, 2015. Towards a Grand Unified Theory of Sports Performance)
Constraints can have spatial or temporal components or both, they reside at all levels of analysis from microscopic to macroscopic, and they operate over a multitude of different time scales, from milliseconds to years.
Actions are not caused by constraints; rather, some actions are excluded by them.