What is the meaning of rigour?

noun

• The quality of being valid and rigorous.
  • synonyms: cogency, validity, rigor
• Something hard to endure.
  • usage: "the asperity of northern winters"
  • synonyms: asperity, grimness, hardship, rigor, severity, severeness, rigorousness, rigourousness
• Excessive sternness.
  • usage: "severity of character"; "the harshness of his punishment was inhuman"; "the rigors of boot camp"
  • synonyms: severity, severeness, harshness, rigor, rigorousness, rigourousness, inclemency, hardness, stiffness