: in a definite way : in a way free of all ambiguity, uncertainty, or obscurity
I will definitely be there.
He's very smart and very funny and is definitely a practical actor. He comes to work to get his job done and to make the best movie possible … Anna Kendrick
Omaha is definitely not hip; green-haired XML programmers do not glide past gourmet coffee emporiums on Razor scooters. Adam Rogers
2
a
—used in speech as a forceful way of expressing agreement or saying "yes"
"Do you think they'll win?" "Definitely!"
b
—used in speech in the phrase definitely not as a forceful way of expressing disagreement or saying "no"