Without doubt; definitely.
"Undoubtedly, she is the most talented musician in the band."
In plain English: Undoubtedly means something is definitely true without any doubt at all.
"Undoubtedly, she will become the best player on the team if she keeps practicing every day."
Undoubtedly comes from Middle English undoutedly, which was formed by adding the suffix -ly to the word undoubted. It entered English as an adverb meaning "without doubt," derived directly from its earlier form that already carried this sense of certainty.