Pivotal, critical (7)
I believe the answer is:
crucial
I believe this is a double definition.
'pivotal' is the first definition.
(similar in meaning)
'critical' is the second definition.
(similar in meaning)
(Other definitions for crucial that I've seen before include "Of paramount importance" , "Of extreme importance" , "Of great importance" , "Of immediate importance" , "All-important" .)