An eccentricity of character or behavior:

"All interesting people have idiosyncrasies."
"Language has its idiosyncrasies, too."

Etymology: Greek idiosynkrasi comprising idios "own, personal" + synkrasis "admixture" from syn "together, with" + krasis "mixture, temperament." Attic Greek "idiotes" meant "private, ordinary person" as opposed to a public figure. However, since this implied that an idiote was unaware of the koina "affairs of the city," he was, by implication, also ignorant, hence the English meaning of "idiot."