The odd one (8)
I believe the answer is:
singular
I believe this is a double definition.
'the odd' is the first definition.
(thesaurus)
'one' is the second definition.
('singular' can be a synonym of 'one')
(Other definitions for singular that I've seen before include "Remarkable; not plural" , "Unusual - not plural" , "distinctive" , "Exceptional, one-off" , "Extraordinary - conspicuous - sole" .)