Eccentricity, idiosyncrasy are not always clearly distinguished when they denote an act, a practice, or a characteristic that impresses the observer as strange or singular.
Eccentricity (compare STRANGE) emphasizes the idea of divergence from the usual or customary; idiosyncrasy implies a following of one’s peculiar temperament or bent especially in trait, trick, or habit; the former often suggests mental aberration, the latter, strong individuality and independence of action.