Tried or tested (6)
I believe the answer is:
proven
I believe this is a double definition.
'tried' is the first definition.
(similar in meaning)
'tested' is the second definition.
(similar in meaning)
(Other definitions for proven that I've seen before include "Established beyond doubt" , "Established with evidence" , "Conclusively demonstrated by evidence" , "Adjective meaning demonstrated beyond doubt" .)