Principal, most important (4)
I believe the answer is:
main
I believe this is a double definition.
'principal' is the first definition.
(both can mean major)
'most important' is the second definition.
(main can mean the most important or major)
(Other definitions for main that I've seen before include "Might on the high seas" , "associated with the Spanish?" , "Chief, primary" , "Principal or high seas" , "Source of water" .)