Solitary, lone (6)
I believe the answer is:
single
I believe this is a double definition.
'solitary' is the first definition.
(similar in meaning)
'lone' is the second definition.
(synonyms)
(Other definitions for single that I've seen before include "Unaccompanied" , "Free" , "Unique" , "One-way ticket" , "Not married, just the one" .)