Incontrovertible evidence (5)

I believe the answer is:
proof
'incontrovertible evidence' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for proof that I've seen before include "Conclusive demonstration" , "Conclusive evidence or verification" , "Positive evidence" , "Render impervious (to)" , "whisky's strength?" .)