a characteristic, habit, mannerism, or the like, that is peculiar to an individual.

the physical constitution peculiar to an individual.

a peculiarity of the physical or the mental constitution, esp. susceptibility toward drugs, food, etc. Compare allergy ( def. 1 ) .

adjective

distinctive , personal , quirky , bizarre , cranky , curious , erratic , freakish , odd , outlandish , peculiar , quaint , queer , singular , strange , unnatural , unusual , weird