'definitely' is an adverb of degree, meaning 'absolutely' or 'without a doubt'. It can be used to emphasize the extent of certainty in a statement or thought.
I definitely don't want that for dinner.
She definitely needs to study more.
The sky was definitely red last night.
though 'definitely' shares similarities with words such as 'certainly' or 'absolutely', it should not be used interchangeably with those words as they have distinct meanings. Additionally, 'definitely' should not be used with linking verbs or verbs of being, as it is not appropriate in those contexts.