1 : in a way that is certain and not able to be doubted : CERTAINLY
This book is unquestionably a masterpiece.
She is unquestionably one of the finest writers who has ever lived.
His strong views have unquestionably influenced others.
2 — used to emphasize that something is definitely true
“Global warming is one of the great challenges of the 21st century.” “Unquestionably.” [=I agree]