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 ) .

noun

affectation , bit , characteristic , distinction , eccentricity , feature , habit , mannerism , peculiarity , singularity , trait , trick , quirk , quirkiness , idiom , oddity