Definition of reasonableness in US English:

reasonableness

noun

  • 1Sound judgment; fairness.

    ‘days which demand wise restraint and calm reasonableness’
    1. 1.1 The quality of being based on good sense.
      ‘he critically evaluated the reasonableness of each assumption’
  • 2The quality of being as much as is appropriate or fair; moderateness.

    ‘disputes about the reasonableness of certain costs’

Pronunciation

reasonableness

/ˈriz(ə)nəb(ə)lnəs//ˈrēz(ə)nəb(ə)lnəs/