Hard Constraints are a type of constraint that must be satisfied in order to reach a feasible solution. Generally, a hard constraint is driven by absolute business necessity, not things that could change with time or additional resources. A hard constraint could be driven by an internal or external influence, but, regardless, it must be accommodated.
Hard constraint examples may include laws, physics, physical or chemical properties, or time limitations (e.g., there are only 24 hours in a day).