2
general::
certainly
used when saying that you are sure about something:
• Amy’s certainly growing, but she’s not gaining weight. • It’s certainly not the cheapest way of getting to New York. • ‘Is this his signature?’ ‘Almost certainly.’
undoubtedly
used when saying that you are completely sure about something. Undoubtedly sounds more certain and a little more formal than certainly:
• They are undoubtedly the better team. • Undoubtedly it will be a huge shock for him. • The furniture, although undoubtedly fashionable, was a little too plain for my taste.
there is no doubt
used when you are completely sure about something:
• There is no doubt that violence on TV influences children. • He’s definitely innocent. There’s no doubt about that.
without (a) doubt
used when you are completely sure about something:
• She was without doubt one of Mexico’s finest artists. • There would be, without a doubt, some perfectly ordinary explanation for his behaviour.
Longman-Thesaurus