The best replacement for constraints is restraints.

The best alternatives to constraints are rated as following:
★★★★★  restraints
★★★★  limitations
★★★  triggers
★★★  criteria
★★  requirements
★★  constrains
★★  time restraints
★  assumptions
★  indexes
★  activity based costing
★  risks
★  constraints
★  business rules
★  barriers
  maximum constraints
  unique constraints
  column constraints
  just in time
  lean systems

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Speaking about constraints, one probably shouldn't forget about generics.

Other search terms related to constraints and their ratings are:
★★★★★ generics
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