1) Absolutely 2) Actually 3) Certainty 4) Certes 5) Doubtless 6) Doubtlessly 7) Indeed 8) Iwis 9) No doubt 10) Ofatruth 11) Surely 12) To be sure 13) Unavoidably 14) Undeniably 15) Undoubtedly 16) Unquestionably 17) Verily 18) Very 19) Wisly 20) Ywis 1) Absolutely 2) Clearly 3) Definitely 4) Definitely or positively 5) Emphatic assent 6) For certain 7) For sure 8) Incontestably 9) Indeed 10) Indisputably 11) Indubitably 12) No problem 13) Of course 14) Really 15) Sure 16) Sure as shooting 17) Sure enough 18) Surely 19) To be sure 20) Undeniably 
• (adv.) Without doubt or question; unquestionably.
Cer'tain·ly adverb Without doubt or question; unquestionably. 
certainly 1. Without any doubt or qualification on the part of the speaker. 2. Used to concede a point that has been made. 3. Used to indicate unreserved assent. 4. Used to indicate emphatic denial or refusal.
with certainty; without doubt; assuredly: I'll certainly be there. · yes, of course: Certainly, take the keys. · surely; to be sure: He certainly is successful.