Definition of "backness"

backness

/bæk.nɪs/

noun
uncountable
1. (phonetics) Quality of being a back vowel.
  Quotations
  The most important aspects of vowel quality are tongue height, frontness/backness and lip rounding.
  1996, Andrew Spencer, Phonology: theory and description, page 25
2. (phonetics) A quality of vowels, indicating the horizontal position of the tongue relative to the back of the mouth (front, near-front, central, near-back, or back).