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Synonyms for Rigorous (same or very similar meaning)

WordNet sense 1 (rigidly accurate; allowing no deviation from a standard):
strict

WordNet sense 2 (marked by strict and particular and complete accordance with fact):
exact

WordNet sense 3 (demanding strict attention to rules and procedures):
stringent, tight

WordNet sense 4 (requiring more than usually expected or thought due; especially great patience and effort and skill):
demanding


From the ODE community, based on WordNetadd/edit