I would define kink as anything that is more than playfully adventurous.
Role-play and costumes would not be kink, IMO, unless they became necessary by one of the participants.
Food play would be of a similar stripe.
S&M would be kink: bondage, cutting, asphyxia, clamps, burning, master/slave.
Scat would be kink.
Multiple participants would be kink.
Animal involvement would be kink.
Mechanical devices would be kink.
Drug involvement (beyond ED or other medical aid) would be kink.
Exhibitionism would be kink.
Wrestling or obsession with bodybuilding, is kink.
Preferring a tatted or pierced or addicted partner would be a preference; needing the partner to be so would be kink.
Needing an unusual location (car sex, workplace sex, tree sex, bike sex) or anything like that, becomes kink.