certainly

Definetely is likely misspelled.

Definetely has no English definition.

As an adverb certainly

is in a way which is certain; with certainty.

Truly is a synonym of certainly.

As adverbs the difference between certainly and truly

is that certainly is in a way which is certain; with certainty while truly is in accordance with the facts; truthfully, accurately.

As adverbs the difference between certainly and unequivocally

is that certainly is in a way which is certain; with certainty while unequivocally is in an unequivocal or unambiguous manner.

Really is a synonym of certainly.

As adverbs the difference between certainly and really

is that certainly is in a way which is certain; with certainty while really is actually; in fact; in reality.

As an interjection really is

indicating surprise at, or requesting confirmation of, some new information; to express skepticism.

Unquestionably is a synonym of certainly.

As adverbs the difference between certainly and unquestionably

is that certainly is in a way which is certain; with certainty while unquestionably is without question; beyond doubt; indubitably.

As adverbs the difference between definetly and certainly

is that definetly is while certainly is in a way which is certain; with certainty.

Positively is a related term of certainly.

As adverbs the difference between positively and certainly

is that positively is (manner)  in a positive manner while certainly is in a way which is certain; with certainty.

As adverbs the difference between certainly and obviously

is that certainly is in a way which is certain; with certainty while obviously is in a obvious manner; clearly apparent.