certainly

As a verb doubt

is (ambitransitive) to lack confidence in; to disbelieve, question, or suspect.

As a noun doubt

is uncertainty, disbelief.

As a adverb certainly is

in a way which is certain; with certainty.

As a noun certainty

is the state of being certain.

As a adverb certainly is

in a way which is certain; with certainty.

As a noun theory

is (obsolete) mental conception; reflection, consideration.

As a adverb certainly is

in a way which is certain; with certainty.

As adverbs the difference between definitely and certainly

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

Certainly is a synonym of sure.

As adverbs the difference between certainly and sure

is that certainly is in a way which is certain; with certainty while sure is without doubt.

As a adjective sure is

physically secure and certain, non-failing, reliable.

As a interjection sure is

yes, of course.

Certainly is a synonym of indeed.

As adverbs the difference between certainly and indeed

is that certainly is in a way which is certain; with certainty while indeed is (modal) truly; in fact; actually.

As a interjection indeed is

indicates emphatic agreement.

Indubitably is a synonym of certainly.

Certainly is a synonym of indubitably.

As adverbs the difference between certainly and indubitably

is that certainly is in a way which is certain; with certainty while indubitably is in a manner that leaves no possibility of doubt; undoubtedly.

As a adverb certainly

is in a way which is certain; with certainty.

As a verb should is

(auxiliary).

As a noun should is

a statement of what should be the case as opposed to what is the case.

Certainly is a derived term of certain.

As a adverb certainly

is in a way which is certain; with certainty.

As a adjective certain is

sure, positive, not doubting.

As a determiner certain is

having been determined but unspecified the quality of some particular subject or object which is known by the speaker to have been specifically singled out among similar entities of its class.

As adverbs the difference between certainly and absolutely

is that certainly is in a way which is certain; with certainty while absolutely is in an absolute or unconditional manner; utterly, positively, wholly {{defdate|first attested around 1350 to 1470}}{{reference-book | last =| first = | authorlink = | coauthors = | editor =brown, lesley | others = | title = the shorter oxford english dictionary | origdate = | origyear = 1933| origmonth = | url = | format = | accessdate = | accessyear = | accessmonth = | edition = 5th | date = | year =2003| month = | publisher =oxford university press | location =oxford, uk | language = | id = | doi = | isbn =978-0-19-860575-7 | lccn = | ol = | pages =9| chapter = | chapterurl = | quote =}}.

As a interjection absolutely is

yes; certainly; expression indicating strong agreement {{defdate|first attested around 1350 to 1470}}.