An idiosyncrasy is an unusual feature of a person (though there are also other uses, see below). It also means odd habit. The term is often used to express eccentricity or peculiarity. A synonym may be quirk.
The term can also be applied to symbols or words. Idiosyncratic symbols mean one thing for a particular person, as a blade could mean war, but to someone else, it could symbolize a surgery. By the same principle, linguists (such as Ferdinand de Saussure) state that words are not only arbitrary, but also largely idiosyncratic signs.