In Portuguese, it means certainly. In Spanish, it can mean either, but I believe certainly is more common. But if I understand it correctly, it's not quite the same as we use certainly in English. It's more like "surely you must be joking" - i.e., not certainly as in truly but relative certainty or something that may be assumed. I welcome corrections from natives, though.
Seguramente is as certain as certainly, but not as certain as surely. The level of intensity of the word's meaning is different than the intensity of surely. Like the difference between maybe vs probably vs surely in English. However, I'm not a native speaker, so I may be wrong.
If I were to say "surely..." in English (though I don't think it's common in California), it would imply that i am making a supposition or asking a question. It definitely implies that I am not certain, whereas "certainly" does imply certainty or at least strong conviction. Which is the meaning of "seguramente"?
This is a good point. However, "surely" does imply certainty and it's usage as an interrogative (e.g. "surely you can't be serious") is colloquial, and is perhaps even being used sarcastically. I would say the most common modern-day usage means mostly certain, but not entirely (e.g. "surely we can agree this is correct"). However, these usages are still colloquial, the proper meaning of the word is synonymous with "certainly". For example, the quotes "Surely God is my salvation; I will trust and not be afraid" and "slowly, but surely".
This gray area exists with seguramente, too.
I think that "certainly" is not necessarily any more definite than "surely" or even "probably."
"Certainly it's true," in context, might really be saying "certainly it must be true?" The point is that written this way, there is still doubt. If you were to write "it is certainly true" that would, on the other hand, say something completely different from "it is probably true."
Context is king.
Wyn, although the two words mean similar things, it would be a bit redundant to say "Certainly it is certain." Americans would usually leave off the first word of your answer. I say usually, because Americans also use "certainly" to mean an affirmative answer, indicating agreement or cooperation, like: (1st person) "Can you be a substitute for Mary at the meeting tonight? Her child is sick." (2nd person) "Certainly, I will be there at seven."
For an answer to a question of validity, Duo's answer is better - "Certainly it is true."
Where I often get mixed up is when to use seguro with ser o estar, because "certain or sure" use es & "safe" uses estar, if I remember correctly, & I did remember THIS time - YAY! ;-)
It is really bizarre that the same word could mean BOTH "probably" AND "certainly"/"surely," which are such very different senses. Other translation programs give the "most probably" meaning, which is different than the "probably" translation given here. I wonder if someone could explain when it is that "seguramente" is used to mean only "probably"