What word refers to a quality or habit that is specific to an individual or group?

The term "idiosyncrasy" refers specifically to a characteristic, habit, or peculiarity that is distinctive to an individual or group. This word emphasizes the unique traits that differentiate a person or group from others, often encompassing behavior, mannerisms, or preferences that are idiosyncratic in nature.

In various contexts—such as psychology, literature, or cultural studies—understanding idiosyncrasies can provide insight into an individual's personality or the cultural nuances of a group. For example, an idiosyncratic behavior might include a particular way of speaking or a specific preference in art that reflects personal taste.

In contrast, the other terms listed do not fit this definition. Indifference refers to a lack of interest or concern, which does not relate to specific qualities. Impetus denotes a force or motivation that causes an action, while introspection involves examining one's own thoughts and feelings, rather than highlighting unique habits or traits. Therefore, the focus on individual or group specificity in the definition makes "idiosyncrasy" the most fitting choice.