Here's why:
* Stress: The stress in "definitely" falls on the second syllable, "fi". This means the first syllable, "def", is unstressed.
* Unstressed Vowel Reduction: In unstressed syllables, vowels often reduce to a schwa sound, which is represented by the symbol ə. In "definitely", the "i" in "def" is reduced to a schwa sound.
So, the unstressed vowel is the "i" in "def", which is pronounced as a schwa.