A back vowel is any in a class of vowel sound used in spoken languages. The defining characteristic of a back vowel is that the highest point of the tongue is positioned relatively back in the mouth without creating a constriction that would be classified as a consonant. Back vowels are sometimes also called dark vowels because they are perceived as sounding darker than the front vowels.[1]
Near-back vowels are essentially a type of back vowels; no language is known to contrast back and near-back vowels based on backness alone.
As here, other back vowels can be transcribed with diacritics of relative articulation applied to letters for neighboring vowels, such as ⟨u̞⟩, ⟨o̝⟩ or ⟨ʊ̠⟩ for a near-close back rounded vowel.
Occurrence
According to PHOIBLE, the most common phonemic back vowel is /u/, occurring in approximately 88% of languages, while the most uncommon phonemic back vowel is /ɒ/, occurring in only 2% of recorded inventories.