In phonetics, vowel reduction is any of various changes in the acoustic quality of vowels as a result of changes in stress, sonority, duration, loudness, articulation, or position in the word (e.g. for the Muscogee language[1]), and which are perceived as "weakening". It most often makes the vowels shorter as well.
Vowels which have undergone vowel reduction may be called reduced or weak. In contrast, an unreduced vowel may be described as full or strong. The prototypical reduced vowel in English is schwa. In Australian English, that is the only reduced vowel, though other dialects have additional ones.